[http-specs] Add http-specs tests for CSV encoding on model properties (#9106)

Adds scenario tests for array encoding on model properties, covering all
four `ArrayEncoding` variants.

## Changes

- **New test folder**: `packages/http-specs/specs/encode/array/`
- **Scenarios added**:
  - `commaDelimited` (CSV) - values joined with `,`
  - `spaceDelimited` (SSV) - values joined with ` `
  - `pipeDelimited` - values joined with `|`
  - `newlineDelimited` - values joined with `\n`

## Example

```tsp
model CommaDelimitedArrayProperty {
  @encode(ArrayEncoding.commaDelimited)
  value: string[];
}

@route("/property/comma-delimited")
@post
op commaDelimited(@Body body: CommaDelimitedArrayProperty): CommaDelimitedArrayProperty;
```

Expected request/response body:
```json
{
  "value": "blue,red,green"
}
```

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>CSV encoding for model properties Scenario
tests</issue_title>
> <issue_description>Add http-specs tests for array encoding, including
all four encode. Add a new folder under
packages/http-specs/specs/encode.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #9029

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tadelesh <1726438+tadelesh@users.noreply.github.com>
Co-authored-by: tadelesh <chenjieshi@microsoft.com>

Author: 3 people

.chronus/changes/copilot-add-csv-encoding-tests-2025-10-28-11-48-49.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/http-specs"
+---
+
+Add tests for array encode.

packages/http-specs/spec-summary.md

@@ -105,6 +105,90 @@ Expected behavior: Should handle nested and combined formatting.
 Test italic text formatting using _single asterisks_.
 Expected behavior: Text between \* should render as italic.
 
+### Encode_Array_Property_commaDelimited
+
+- Endpoint: `post /encode/array/property/comma-delimited`
+
+Test operation with request and response model contains a string array property with commaDelimited encode.
+Expected request body:
+
+```json
+{
+  "value": "blue,red,green"
+}
+```
+
+Expected response body:
+
+```json
+{
+  "value": "blue,red,green"
+}
+```
+
+### Encode_Array_Property_newlineDelimited
+
+- Endpoint: `post /encode/array/property/newline-delimited`
+
+Test operation with request and response model contains a string array property with newlineDelimited encode.
+Expected request body:
+
+```json
+{
+  "value": "blue\nred\ngreen"
+}
+```
+
+Expected response body:
+
+```json
+{
+  "value": "blue\nred\ngreen"
+}
+```
+
+### Encode_Array_Property_pipeDelimited
+
+- Endpoint: `post /encode/array/property/pipe-delimited`
+
+Test operation with request and response model contains a string array property with pipeDelimited encode.
+Expected request body:
+
+```json
+{
+  "value": "blue|red|green"
+}
+```
+
+Expected response body:
+
+```json
+{
+  "value": "blue|red|green"
+}
+```
+
+### Encode_Array_Property_spaceDelimited
+
+- Endpoint: `post /encode/array/property/space-delimited`
+
+Test operation with request and response model contains a string array property with spaceDelimited encode.
+Expected request body:
+
+```json
+{
+  "value": "blue red green"
+}
+```
+
+Expected response body:
+
+```json
+{
+  "value": "blue red green"
+}
+```
+
 ### Encode_Bytes_Header_base64
 
 - Endpoint: `get /encode/bytes/header/base64`

packages/http-specs/specs/encode/array/main.tsp

@@ -0,0 +1,112 @@
+import "@typespec/http";
+import "@typespec/spector";
+
+using Http;
+using Spector;
+
+@doc("Test for encode decorator on array.")
+@scenarioService("/encode/array")
+namespace Encode.Array;
+
+model CommaDelimitedArrayProperty {
+  @encode(ArrayEncoding.commaDelimited)
+  value: string[];
+}
+
+model SpaceDelimitedArrayProperty {
+  @encode(ArrayEncoding.spaceDelimited)
+  value: string[];
+}
+
+model PipeDelimitedArrayProperty {
+  @encode(ArrayEncoding.pipeDelimited)
+  value: string[];
+}
+
+model NewlineDelimitedArrayProperty {
+  @encode(ArrayEncoding.newlineDelimited)
+  value: string[];
+}
+
+@route("/property")
+namespace Property {
+  @route("/comma-delimited")
+  @scenario
+  @scenarioDoc("""
+    Test operation with request and response model contains a string array property with commaDelimited encode.
+    Expected request body:
+    ```json
+    {
+      "value": "blue,red,green"
+    }
+    ```
+    Expected response body:
+    ```json
+    {
+      "value": "blue,red,green"
+    }
+    ```
+    """)
+  @post
+  op commaDelimited(@body body: CommaDelimitedArrayProperty): CommaDelimitedArrayProperty;
+
+  @route("/space-delimited")
+  @scenario
+  @scenarioDoc("""
+    Test operation with request and response model contains a string array property with spaceDelimited encode.
+    Expected request body:
+    ```json
+    {
+      "value": "blue red green"
+    }
+    ```
+    Expected response body:
+    ```json
+    {
+      "value": "blue red green"
+    }
+    ```
+    """)
+  @post
+  op spaceDelimited(@body body: SpaceDelimitedArrayProperty): SpaceDelimitedArrayProperty;
+
+  @route("/pipe-delimited")
+  @scenario
+  @scenarioDoc("""
+    Test operation with request and response model contains a string array property with pipeDelimited encode.
+    Expected request body:
+    ```json
+    {
+      "value": "blue|red|green"
+    }
+    ```
+    Expected response body:
+    ```json
+    {
+      "value": "blue|red|green"
+    }
+    ```
+    """)
+  @post
+  op pipeDelimited(@body body: PipeDelimitedArrayProperty): PipeDelimitedArrayProperty;
+
+  @route("/newline-delimited")
+  @scenario
+  @scenarioDoc("""
+    Test operation with request and response model contains a string array property with newlineDelimited encode.
+    Expected request body:
+    ```json
+    {
+      "value": "blue\\nred\\ngreen"
+    }
+    ```
+    Expected response body:
+    ```json
+    {
+      "value": "blue\\nred\\ngreen"
+    }
+    ```
+    """)
+  @post
+  op newlineDelimited(@body body: NewlineDelimitedArrayProperty): NewlineDelimitedArrayProperty;
+}

packages/http-specs/specs/encode/array/mockapi.ts

@@ -0,0 +1,43 @@
+import { json, passOnSuccess, ScenarioMockApi } from "@typespec/spec-api";
+
+export const Scenarios: Record<string, ScenarioMockApi> = {};
+
+const colors = ["blue", "red", "green"];
+
+function createPropertyServerTests(uri: string, delimiter: string) {
+  const encodedValue = colors.join(delimiter);
+  return passOnSuccess({
+    uri,
+    method: "post",
+    request: {
+      body: json({
+        value: encodedValue,
+      }),
+    },
+    response: {
+      status: 200,
+      body: json({ value: encodedValue }),
+    },
+    kind: "MockApiDefinition",
+  });
+}
+
+Scenarios.Encode_Array_Property_commaDelimited = createPropertyServerTests(
+  "/encode/array/property/comma-delimited",
+  ",",
+);
+
+Scenarios.Encode_Array_Property_spaceDelimited = createPropertyServerTests(
+  "/encode/array/property/space-delimited",
+  " ",
+);
+
+Scenarios.Encode_Array_Property_pipeDelimited = createPropertyServerTests(
+  "/encode/array/property/pipe-delimited",
+  "|",
+);
+
+Scenarios.Encode_Array_Property_newlineDelimited = createPropertyServerTests(
+  "/encode/array/property/newline-delimited",
+  "\n",
+);