fix(mcp): tolerate invalid tool output schemas

[nicolascancino]

Issue for this PR

Closes #26529

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Makes MCP tool loading resilient when a server returns a valid tools/list response whose outputSchema contains refs the SDK cannot resolve.

The normal client.listTools() path still runs first. If it fails with a schema reference error, OpenCode retries tools/list with a tolerant result schema that keeps inputSchema validation and treats outputSchema as opaque.

This fixes Google Stitch-style failures where the MCP transport connects but OpenCode marks the server as failed while validating tool output schemas.

How did you verify your code works?

cd packages/opencode
bun test test/mcp/lifecycle.test.ts --timeout 30000

Result:

20 pass
0 fail
51 expect() calls
Ran 20 tests across 1 file. [2.13s]

Screenshots / recordings

N/A — non-UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR