fix(core): handle unsupported JSON Schema versions in MCP tool validation

[afarber]

Summary

MCP servers built with Rust rmcp v0.9.1+ send JSON Schema draft-2020-12, which AJV (the JSON validator) doesn't support by default. This causes tool invocations to fail with "no schema with key or ref" errors. This fix wraps schema compilation in a try-catch, skipping client-side validation when the schema version is unsupported rather than blocking the tool call.

Details

• Applied the same lenient validation pattern already used in LenientJsonSchemaValidator (mcp-client.ts) for MCP output schemas
• When ajValidator.compile() throws (unsupported schema version, invalid $ref, etc.), skip validation and let the MCP server validate parameters instead
• Works with any JSON Schema version (draft-04, 06, 07, 2019-09, 2020-12, future versions)
• No new dependencies required

Related Issues

Fixes #14970 Related to https://github.com/modelcontextprotocol/rust-sdk/issues/587

How to Validate

1. Run unit tests: npm run test -- packages/core/src/utils/schemaValidator.test.ts
2. Build an rmcp MCP server (e.g., from https://github.com/modelcontextprotocol/rust-sdk):

     cd ~/rust-sdk
     cargo build --example servers_counter_stdio -p mcp-server-examples

3. Configure in ~/.gemini/settings.json: {"mcpServers": {"counter": {"command": "/path/to/servers_counter_stdio"}}}
4. Run gemini, then /mcp schema should show tools without errors
5. Invoke a tool like sum - should succeed instead of failing with schema error

Pre-Merge Checklist

• [ ] Updated relevant documentation and README (if needed)
• [x] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [ ] Validated on required platforms/methods:
  • [x] MacOS
    • [x] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [ ] Linux
    • [ ] npm run
    • [ ] npx
    • [ ] Docker