fix: improve MCP version mismatch detection and error messages

[zemaj]

Summary

This PR addresses issue #292 by improving MCP version mismatch detection and error messages in the rmcp-client handshake process.

Changes

• Enhanced error messages: All handshake errors now include the expected MCP protocol version (MCP_SCHEMA_VERSION) to help users quickly identify version compatibility issues
• Explicit version validation: After successful handshake conversion, the client now explicitly compares the server's protocol version with the expected version
• Clear mismatch error: When versions don't match, users receive a clear, actionable error message explaining the mismatch and suggesting they ensure compatibility
• Test coverage: Added a unit test to verify MCP_SCHEMA_VERSION is accessible and properly formatted

Technical Details

Modified code-rs/rmcp-client/src/rmcp_client.rs:

• Import MCP_SCHEMA_VERSION from mcp_types
• Update handshake error messages to include expected protocol version
• Add explicit version comparison after handshake (lines 169-176)
• Add type annotation to initialize_result for proper type inference
• Add test module with version constant validation

Testing

• ✅ ./build-fast.sh passes
• ✅ Added unit test for MCP_SCHEMA_VERSION format validation

Risk Assessment

Low risk: Changes are minimal and defensive:

• Only affects error messages and adds a new validation check
• Does not modify existing behavior for successful handshakes with matching versions
• Early failure (explicit version check) is better than silent mismatch
• Type annotation fix is a compile-time improvement

Related Issue

Fixes #292

🤖 Generated with Claude Code

[zemaj]

Closing in favor of PR #361, which supersedes these changes and includes the schema-format test from this branch.