fix(mcp): handle empty 202 notification acknowledgements

[G7CNF]

Fixes #1436.\n\nMCP Streamable HTTP notifications may legitimately return 202 Accepted with an empty body. The HTTP transport now treats that as an empty JSON-RPC acknowledgement instead of trying to parse JSON, which unblocks session initialization against servers such as kubernetes-mcp-server.\n\nValidation:\n- cargo test -p ironclaw --lib http_transport::tests::test_accepted_notification_returns_empty_response -- --nocapture\n- cargo clippy -p ironclaw --lib --all-features -- -D warnings

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical issue where the MCP Streamable HTTP notifications failed to process 202 Accepted responses when the response body was empty. The change ensures that such responses are now correctly treated as successful acknowledgements, thereby resolving a blocker for session initialization with certain MCP servers.

Highlights

• HTTP Transport Handling: The HTTP transport for MCP notifications was updated to correctly interpret a 202 Accepted HTTP status code with an empty response body as a valid, empty JSON-RPC acknowledgement. Previously, this would cause parsing errors.
• New Test Case: A new asynchronous test was added to validate the correct handling of 202 Accepted responses, ensuring that session initialization against servers like kubernetes-mcp-server is unblocked.

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[gemini-code-assist]

Code Review

This pull request fixes an issue where the MCP HTTP transport would fail when a server returns a 202 Accepted response with an empty body, which is a valid acknowledgement for notifications. The change correctly handles this case by creating a default success response. A new test is added to verify this behavior.

My review includes suggestions to improve the new test code by using an existing helper function for creating requests and by consistently using expect() instead of unwrap() for better error reporting, in line with repository guidelines.