fix: Always put the well-known endpoints at the server root

# Bug Report: Incorrect `.well-known/oauth-protected-resource` endpoint path when `resource_server_url` ends with `/sse`

## Summary
When configuring FastMCP with OAuth2 authentication and setting the `resource_server_url` to end with `/sse` (as required by VSCode MCP clients), FastMCP incorrectly serves the `.well-known/oauth-protected-resource` endpoint at `/sse/.well-known/oauth-protected-resource` instead of the expected root path `/.well-known/oauth-protected-resource`.

## Environment
- **FastMCP version**: Part of `mcp` Python library (check with `pip show mcp`)
- **Python version**: 3.13
- **Operating System**: macOS
- **MCP Client**: VSCode with MCP extension
- **Affected Files**: 
  - `mcp/server/fastmcp/server.py` (lines ~790-797)
  - `mcp/server/auth/routes.py` (lines ~215-224)

## Expected Behavior
1. The `.well-known/oauth-protected-resource` endpoint should always be served at the root path (`/.well-known/oauth-protected-resource`) regardless of the `resource_server_url` configuration
2. The `resource` field in the `.well-known` response should point to the actual protected resource (e.g., `/sse`)
3. OAuth2 discovery should work correctly with MCP clients like VSCode

## Actual Behavior
When `resource_server_url` is set to `http://localhost:8099/sse`, FastMCP:
1. Serves `.well-known/oauth-protected-resource` at `/sse/.well-known/oauth-protected-resource`
2. The SSE endpoint `/sse` returns a `www-authenticate` header with `resource_metadata="http://localhost:8099/sse/.well-known/oauth-protected-resource"`
3. OAuth2 discovery fails because clients expect the `.well-known` endpoint at the root

## Steps to Reproduce

1. Create a FastMCP server with OAuth2 configuration:

```python
from mcp.server.fastmcp import FastMCP
from mcp.server.auth.settings import AuthSettings
from pydantic import AnyHttpUrl

# Configure auth settings with /sse endpoint
auth_settings = AuthSettings(
    issuer_url=AnyHttpUrl("https://login.microsoftonline.com/tenant-id/v2.0"),
    resource_server_url=AnyHttpUrl("http://localhost:8099/sse"),  # Note: ends with /sse
    required_scopes=["https://example.com/scope"]
)

mcp = FastMCP(
    "Test Server",
    token_verifier=your_token_verifier,
    auth=auth_settings,
)

app = mcp.sse_app()
```

2. Start the server: `uvicorn server:app --port 8099`

3. Test the endpoints:
```bash
# This should work but returns 404
curl http://localhost:8099/.well-known/oauth-protected-resource

# This works but shouldn't be the location
curl http://localhost:8099/sse/.well-known/oauth-protected-resource

# SSE endpoint references wrong .well-known location
curl -I http://localhost:8099/sse
# Returns: resource_metadata="http://localhost:8099/sse/.well-known/oauth-protected-resource"
```

## Root Cause Analysis

**Exact Location of Bug**: 
- **File**: `mcp/server/fastmcp/server.py` 
- **Lines**: ~790-797 in the `sse_app()` method
- **Function**: `FastMCP.sse_app()`

**The Issue**: When setting up OAuth2 authentication, FastMCP constructs the `resource_metadata_url` incorrectly:

```python
# BUGGY CODE - Line ~790-797 in sse_app() method
resource_metadata_url = AnyHttpUrl(
    str(self.settings.auth.resource_server_url).rstrip("/") + "/.well-known/oauth-protected-resource"
)
```

When `resource_server_url` is `http://localhost:8099/sse`, this creates `http://localhost:8099/sse/.well-known/oauth-protected-resource`.

However, the actual `.well-known` endpoint is created by `create_protected_resource_routes()` (in `mcp/server/auth/routes.py` lines ~215-224), which always creates it at the root path:

```python
# CORRECT CODE - This always creates /.well-known/oauth-protected-resource at root
return [
    Route(
        "/.well-known/oauth-protected-resource",
        endpoint=cors_middleware(handler.handle, ["GET", "OPTIONS"]),
        methods=["GET", "OPTIONS"],
    )
]
```

**The Fix**: The `resource_metadata_url` should be constructed from the base URL, not the `resource_server_url`:

```python
# PROPOSED FIX
if self.settings.auth and self.settings.auth.resource_server_url:
    from pydantic import AnyHttpUrl
    from urllib.parse import urlparse
    
    # Extract base URL from resource_server_url
    parsed = urlparse(str(self.settings.auth.resource_server_url))
    base_url = f"{parsed.scheme}://{parsed.netloc}"
    
    resource_metadata_url = AnyHttpUrl(
        base_url + "/.well-known/oauth-protected-resource"
    )
```

## Impact
- **High**: Breaks OAuth2 discovery for MCP clients like VSCode
- MCP servers cannot be properly authenticated when using the recommended `/sse` resource URL pattern
- Workarounds require custom endpoint overrides, defeating the purpose of built-in auth support

## Proposed Solution
The `.well-known/oauth-protected-resource` endpoint should always be served at the root path (`/.well-known/oauth-protected-resource`), regardless of the `resource_server_url` configuration. The `resource_server_url` should only affect:
1. The `resource` field value in the `.well-known` response
2. The `resource_metadata` reference in `www-authenticate` headers

## Current Workaround
Override the built-in `.well-known` endpoint with a custom implementation:

```python
async def custom_well_known_endpoint(request):
    return JSONResponse({
        "resource": f"{config.EXTERNAL_ADDRESS}/sse",
        "authorization_servers": ["https://login.microsoftonline.com/tenant/v2.0"],
        "scopes_supported": ["https://example.com/scope"],
        "bearer_methods_supported": ["header"]
    })

# Override the built-in endpoint
app.router.routes.insert(0, Route("/.well-known/oauth-protected-resource", custom_well_known_endpoint, methods=["GET"]))
```

## Additional Context
- This issue specifically affects integration with VSCode MCP clients, which require the resource URL to end with `/sse`
- The OAuth2 specification (RFC 8414) defines `.well-known` endpoints should be at predictable root paths
- Other OAuth2 implementations (e.g., Auth0, Okta) serve `.well-known` endpoints at root regardless of resource configuration

## Related Documentation
- [RFC 8414 - OAuth 2.0 Authorization Server Metadata](https://tools.ietf.org/html/rfc8414)
- [MCP OAuth2 Authentication Documentation](https://modelcontextprotocol.io/docs/concepts/authentication)

Author: seanhoughton

src/mcp/server/fastmcp/server.py

@@ -792,11 +792,15 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):
             resource_metadata_url = None
             if self.settings.auth and self.settings.auth.resource_server_url:
                 from pydantic import AnyHttpUrl
-
+                from urllib.parse import urlparse
+                
+                # Extract base URL from resource_server_url
+                parsed = urlparse(str(self.settings.auth.resource_server_url))
+                base_url = f"{parsed.scheme}://{parsed.netloc}"
+                
                 resource_metadata_url = AnyHttpUrl(
-                    str(self.settings.auth.resource_server_url).rstrip("/") + "/.well-known/oauth-protected-resource"
+                    base_url + "/.well-known/oauth-protected-resource"
                 )
-
             # Auth is enabled, wrap the endpoints with RequireAuthMiddleware
             routes.append(
                 Route(