feat(unstable-v2): define version mismatch errors

[benbrandt]

Summary

• Define v2 negotiation as selecting the highest supported version at or below the requested version.
• Add -32022 (Unsupported protocol version) with required requested and supported error data when no supported version qualifies.
• Keep malformed version values as -32602, require clients to check successful selections, and leave mismatched connections uninitialized.
• Expand dual-stack migration guidance with the runnable TypeScript example, Rust routing, client fallback, and adapter boundaries.

Compatibility

This changes the v2 specification and error catalog only. The v1 specification, schema source, and generated artifacts are unchanged.

A v1 client reaching a v2-only endpoint receives an ordinary initialization error. The endpoint does not need to manufacture v2 client identity or invoke its typed v2 handler to report the mismatch. Legacy v1-only agents retain their existing negotiation contract.

Companion SDK draft PRs:

• TypeScript: fix(v2): align protocol version negotiation typescript-sdk#286
• Rust: fix(acp): align protocol version negotiation rust-sdk#424

Validation

• npm run generate
• npm run check
• git diff --check
• Verified no v1 protocol/schema changes against current main.

The branch is rebased onto current upstream main. The Mintlify site was not built.