fix(openapi): preserve anyOf unions in MCP tool input schemas

[syf2211]

Summary

Stop injecting a single top-level "type" into MCP tool parameter schemas when anyOf is already present. This fixes non-null union types (e.g. dict | list) being collapsed to one arbitrary type and rejected by the MCP server's JSON Schema validator.

Motivation

For request body fields typed as T | U where both types are non-null, convert_openapi_to_mcp_tools was adding a top-level "type" picked arbitrarily from the union. The MCP jsonschema validator then rejected inputs matching the other branch with misleading errors like is not of type 'array'.

Fixes #307

Changes

File	Change
fastapi_mcp/openapi/convert.py	Only inject "type" when neither "type" nor "anyOf" is present (path, query, and body params)
tests/test_openapi_conversion.py	Add regression test for dict | list union; update optional-param assertions to expect preserved anyOf

Tests

• uv run pytest tests/test_openapi_conversion.py --no-cov -q — 11 passed
• Composer-2.5 review: APPROVE after test updates

Notes

• Also affects nullable Optional[T] schemas (anyOf: [T, null]), which is the correct JSON Schema representation and matches MCP validator behavior.
• Related issues: convert_openapi_to_mcp_tools adds "type" field, breaking nullable schemas #246, [BUG] Union types with arrays generate invalid MCP schemas (missing 'items' property) #165, [BUG] MCP Tool input_schema not valid JSON Schema 2020-12; OpenAPI fields leak into tool schemas #218

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[K4bain]

@syf2211 — heads up: this PR and #308 address the same root cause (sibling type injected next to anyOf). I adopted the #308 variant as #337 since it extends the guard to oneOf/allOf as well — your analysis and test expectations were a great cross-check and you are credited in the thread there. Either way the bug you found is real and hopefully one of these lands soon.