feat!: add server discovery and negotiation (SEP-2575)

[DaleSeo]

Motivation and Context

This implements the discovery and negotiation portion of SEP-2575. It adds the server/discover RPC, per-request protocol version negotiation, and typed unsupported-version errors so clients can learn a server's versions and capabilities without initializing a session, while preserving the legacy initialization flow.

All six protocol union enums generated by ts_union! are now #[non_exhaustive]. This requires downstream exhaustive matches to add a wildcard arm once, but allows future protocol variants to be added without causing the same Rust API break.

How Has This Been Tested?

Added tests

Breaking Changes

Yes. ClientRequest, ClientNotification, ClientResult, ServerRequest, ServerNotification, and ServerResult are now #[non_exhaustive], so downstream exhaustive matches must add a wildcard arm. DiscoverRequest and DiscoverResult also add variants to ClientRequest and ServerResult; subsequent variants can be added without another exhaustive-match break.

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