Export diagnostic-collecting API for downstream emitter composition

[Copilot]

The emitter was directly reporting diagnostics to the program, preventing downstream emitters from collecting and handling diagnostics according to TypeSpec best practices ([Result, readonly Diagnostic[]] pattern).

Changes

• createModel() - Now returns [CodeModel, readonly Diagnostic[]] instead of CodeModel
• createCodeModel() - New exported function wrapping emission logic, returns [void, readonly Diagnostic[]]
• Logger - Added diagnostic collection mode via constructor parameter and getDiagnostics() method
• $onEmit() - Refactored to call createCodeModel() and report collected diagnostics

Usage

import { createCodeModel } from "@typespec/http-client-csharp";

export async function $onEmit(context: EmitContext<MyOptions>) {
  const [, diagnostics] = await createCodeModel(context);
  // Handle diagnostics as needed
  context.program.reportDiagnostics(diagnostics);
}

The Logger class now stores diagnostics when instantiated with collectDiagnostics: true, enabling the pattern above while maintaining backward compatibility for normal emission.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[http-client-csharp] export an API for downstream emitter to call</issue_title>
<issue_description>Originally posted: Azure/azure-sdk-for-net#54788

Per best practice of writing an emitter or typespec library, we should have signatures like this:

getStuff(): [Stuff, readonly Diagnostic[]]

Therefore here, we are making the following changes:

1. in emitter.ts file, wrap everything inside the if (!program.compilerOptions.noEmit && !program.hasError()) into a new function, which returns [void, readonly Diagnostic[]].
2. refactor the createCodeModel method to make it return [CodeModel, readonly Diagnostic[]] instead. Inside the createCodeModel method, see if there are code snippets calling the reportDiagnostic method, we need to change them to append the diagnostic into the array and return it.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [http-client-csharp] export an API for downstream emitter to call #9316

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