fix: consume response body on unexpected content type to prevent client hang

[vishal-vanam]

Summary

Fixes #2432

When an MCP server returns an unexpected Content-Type (e.g. text/plain instead of application/json or text/event-stream), session.initialize() hangs indefinitely because the response body is never consumed, causing the httpx streaming context to block on exit.

The fix: Call await response.aread() to consume the response body before sending the error message through the read stream. This allows the streaming context to close cleanly and the error to propagate to the caller.

Changes

• src/mcp/client/streamable_http.py: Added await response.aread() in the unexpected content-type branch to drain the response body
• tests/shared/test_streamable_http.py: Added test using httpx MockTransport to verify the client raises MCPError instead of hanging

Test plan

• New test test_unexpected_content_type_does_not_hang passes locally
• Verify existing streamable HTTP tests still pass