fix(openapi): preserve array items for Optional[List] MCP tool params

[syf2211]

Summary

Fix invalid MCP tool input schemas for Optional[List[X]] request body and query parameters. These previously exposed {type: "array"} without an items field after OpenAPI anyOf composition was simplified.

Motivation

Fixes #304. MCP clients (VS Code Copilot, Claude Code) reject tool schemas missing items on array-typed parameters with errors like: tool parameters array type must have items.

Changes

• Add _hoist_array_fields_from_composition() to copy items (and type) from anyOf/oneOf array variants before composition keys are removed in clean_schema_for_display()
• Apply clean_schema_for_display() to each path/query/body property when building MCP tool inputSchema
• Add regression tests for hoist logic and Optional[List[str]] request body conversion

Tests

uv run pytest -q
# 119 passed, 83% coverage

uv run ruff check fastapi_mcp/openapi/utils.py fastapi_mcp/openapi/convert.py tests/test_openapi_conversion.py
# All checks passed

Notes

• Composer-2.5 review: APPROVE
• Optional scalar anyOf params are simplified to a single type; optionality remains expressed via the required list (existing behavior).
• oneOf array variants are handled by the same hoist logic; only anyOf is covered by a dedicated unit test.

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