fix: negotiate protocol version in handler

[actsalan]

Summary

Fixes #916 - ensures context.protocol_version() returns the negotiated version in all transports and modes.

Motivation and Context

Three related bugs existed in protocol version handling:

1. Stateless HTTP: serve_directly passed None as peer_info, so context.protocol_version() was always None in handlers
2. Stateful/stdio: serve_server negotiated the version in the initialize response but never updated peer_info to match, so handlers saw the client's raw requested version instead of the negotiated one
3. Default handler: The default ServerHandler::initialize didn't apply negotiation logic, so the response could echo an unnegotiated version

How Has This Been Tested?

• Added tests for both stdio (test_protocol_version_negotiation.rs) and stateless HTTP (test_stateless_protocol_version.rs) transports
• Tests verify that all KNOWN_VERSIONS are correctly echoed back to clients
• Tests verify that unknown versions fall back to LATEST
• All existing tests continue to pass

Breaking Changes

None. This fix maintains backward compatibility while ensuring correct protocol version negotiation.

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

Changes made:

• tower.rs: Reconstruct peer_info per-request from MCP-Protocol-Version header for stateless mode
• service/server.rs: Update peer_info after negotiation so subsequent handlers see the negotiated version
• handler/server.rs: Apply negotiate_protocol_version in default initialize response