fix(mcp): fall back on circular tool schemas

[MaheshBhushan]

Issue for this PR

Closes #16899

Type of change

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

What does this PR do?

This prevents session crashes when an MCP tool exposes input JSON Schema with circular $defs references (for example nested _And references).

Before this change, we passed those schemas directly into jsonSchema(...), and downstream schema resolution could recurse until failure (RecursionError: maximum recursion depth exceeded).

Now we detect circular $defs reference graphs and, for those tools only, fall back to a permissive object input schema:

• { type: "object", additionalProperties: true }

This keeps the tool available and avoids hard crashing the session processor.

How did you verify your code works?

• Added circular-ref detection over $defs reference graph (#/$defs/...).
• Verified normal non-circular schemas still use the original transformed schema path.
• Verified circular schemas use fallback path and emit a warning log.

Screenshots / recordings

Not included (core logic fix; no UI changes).

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

The searches found PR #16910 (the current PR) and a couple of related schema-handling PRs, but those are from 2024 and address different issues (Gemini schema compatibility and $ref expansion). They don't appear to be addressing the same circular reference detection issue for MCP tools.

No duplicate PRs found

[MaheshBhushan]

quick note on the failing ypecheck check: the reported errors are in packages/desktop/src/index.tsx and packages/desktop/src/updater.ts, while this PR only changes packages/opencode/src/session/prompt.ts.

so this appears unrelated to the scoped MCP schema change in this PR (likely baseline/dev-branch issue). happy to rebase/re-run once upstream desktop typecheck is green again.