Include response body in HttpRequestException for transport client errors

[Copilot]

EnsureSuccessStatusCode throws without the response body, making server error diagnostics difficult when the body contains error details (e.g., 400 Bad Request with validation messages).

Changes

• New HttpResponseMessageExtensions in src/Common/:

  • EnsureSuccessStatusCodeWithResponseBodyAsync() - reads body before throwing (with 5-second timeout to prevent hangs)
  • CreateHttpRequestException() - constructs exception with body in message
  • Catches all exceptions when reading response body (including cancellation) to ensure the HTTP error is always thrown
  • Truncates large response bodies to 1KB for readability

• Updated transport clients to use the new method:

  • SseClientSessionTransport (connect + send)
  • StreamableHttpClientSessionTransport (send)
  • ClientOAuthProvider (token exchange + metadata fetch)

Result

Before:

HttpRequestException: Response status code does not indicate success: 400 (Bad Request).

After:

HttpRequestException: Response status code does not indicate success: 400 (Bad Request). Response body: {"error":"invalid_grant","error_description":"Authorization code expired"}

HttpRequestException.StatusCode is preserved on .NET targets.

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[halter73]

Should we trim the body if it's too large for readability in the normal case where the error body isn't that interesting and just represents an outage or something like that?

[stephentoub]

@copilot, please address @halter73's comments

[Copilot]

Should we trim the body if it's too large for readability in the normal case where the error body isn't that interesting and just represents an outage or something like that?

Added trimming - response bodies are now limited to 8KB max, with "..." appended if truncated. (1d7fe0c)