Return HTTP status code 202 for notification initialized method

[siddhantbajaj]

Brief summary: Return 202 Accepted (no body) for POSTed JSON-RPC notifications over Streamable HTTP, specifically notifications/initialized, to align with the MCP spec. Added a focused handler and unit test.

Motivation and Context

• The MCP Streamable HTTP spec requires servers to return 202 Accepted with no body for JSON-RPC notifications/responses. Previously, notifications/initialized returned 200.
• This change brings behavior into compliance and prevents client-side mismatches.
• Spec reference: Model Context Protocol Transports – Backwards Compatibility.

How Has This Been Tested?

• Added a unit test in test/mcp/server/transports/streamable_http_transport_test.rb:
• Verifies POST to notifications/initialized returns HTTP 202 with empty headers and empty body.
• Existing transport tests cover non-notification flows.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[koic]

@siddhantbajaj Thanks! Can you squash commits into one?