Give recursive tool output schemas an object root (#3337)

[AuraVox-ro]

Fixes #3337

Recursive BaseModel tool return types made Pydantic emit a $ref-only root with no type: object. That serializes on 2026-07-28, but 2025-11-25 Tool.outputSchema requires an object root, so a legacy tools/list failed validation for the entire listing.

This inlines the referenced object definition at the root and keeps $defs so nested self-references still resolve.

Motivation and Context

See #3337. One recursive tool schema should not fail the entire tools/list for 2025-11-25 sessions.

How Has This Been Tested?

• unit: test_structured_output_recursive_basemodel
• integration: test_tool_recursive_output_schema_lists_on_legacy (Client mode=legacy plus a second unrelated tool)

Breaking Changes

None. Recursive output schemas gain a root type: object; nested $refs are unchanged.

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 am assigned to the linked issue (or it is labeled help wanted, or I'm a maintainer)
• I have disclosed any AI assistance and can explain the change in my own words
• 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

Additional context

AI-assisted. I own the change and can answer review questions.

[github-actions]

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