fix: negotiate stateless initialize versions

[DaleSeo]

Fixes #1079.

Motivation and Context

Stateless Streamable HTTP dispatches each request with serve_directly_with_ct, bypassing the initialization post-processing used by stdio and stateful HTTP. The default ServerHandler::initialize negotiates versions itself, but an override replaces that default. Before this PR, such an override could return the server's latest version even when the client requested another version the SDK supports.

This PR puts the invariant at the stateless transport boundary. A private Service wrapper records the requested initialize version, delegates to the handler, negotiates any InitializeResult with the existing helper, and synchronizes peer info. Both stateless direct-dispatch entry points use the wrapper, so JSON and SSE response framing receive the same corrected result.

How Has This Been Tested?

Added tests

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

[Copilot]

Pull request overview

This PR fixes stateless Streamable HTTP initialization so that protocol version negotiation is enforced at the transport boundary even when a ServerHandler overrides initialize, aligning behavior with stdio and stateful HTTP and satisfying the lifecycle requirement to echo a requested known protocol version.

Changes:

• Introduces a private NegotiatingStatelessHttpService wrapper that post-processes InitializeResult with negotiate_protocol_version and synchronizes the stored peer info protocol version.
• Wraps both stateless direct-dispatch entry points in streamable_http_server/tower.rs so JSON and SSE framing both return the negotiated version.
• Updates/extends stateless protocol-version tests to cover handlers that override initialize, for both JSON and SSE responses, plus unknown-version fallback preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/rmcp/src/transport/streamable_http_server/tower.rs	Adds a stateless-only service wrapper to negotiate InitializeResult::protocol_version and keep peer info consistent, applied to both direct-dispatch paths.
crates/rmcp/tests/test_stateless_protocol_version.rs	Adds regression coverage for overridden initialize across JSON and SSE stateless responses, and verifies unknown-version fallback behavior.