fix(openapi): preserve array items for Optional[List] MCP tool params (adopted from #314)

[K4bain]

Adopted from #314 (original patch by @syf2211) — rebased onto current main and verified.

Why this is needed

A request body field like tags: Optional[List[str]] resolves (after expansion + anyOf cleanup) to a property that carries only anyOf variants and no top-level items — so MCP clients/LLMs see an array param with no item schema and cannot construct valid input.

This PR:

1. Adds _hoist_array_fields_from_composition(): before clean_schema_for_display() strips anyOf/oneOf, it copies items (and type=array) from the first array variant up to the top level.
2. Runs converted properties through clean_schema_for_display() so the cleaned shape is what clients actually receive.

Result: Optional[List[str]] becomes a plain {type: array, items: {type: string}} tool param.

Verification

• New tests: unit test for the hoisting on a hand-built anyOf schema, plus an end-to-end FastAPI app test asserting tags resolves to type array + items string with no anyOf left. Both fail on main, pass with the fix.
• Full unit suite: 71 passed (69 existing + 2 new).

Credit to @syf2211 for the original report and patch in #314.

[K4bain]

Closing for housekeeping - the repo had no reviewer engagement and we are re-approaching these areas through a narrower pipeline. Happy to reopen on request.