fix: restore LEANN HTTP server OpenAPI and search body parsing

[Paltidor87]

What does this PR do?

Fixes LEANN HTTP server schema/validation issues that broke both /openapi.json and POST /indexes/{index_name}/search.

Changes

• Add a regression test module for HTTP server behavior:
  • test_openapi_schema_loads ensures /openapi.json renders successfully.
  • test_search_endpoint_accepts_json_body ensures plain JSON request bodies are accepted by the search route.
• Update packages/leann-core/src/leann/server.py:
  • Import Body from FastAPI in lazy loader.
  • Change search route signature from typed local model body to body: dict[str, Any] = Body(...).
  • Parse/validate with SearchRequest(**body) inside the route handler.
  • Keep response model + search behavior unchanged.

This removes the unresolved forward-ref/schema generation failure and restores normal request parsing.

Related Issues

Fixes #

Checklist

• Tests pass (uv run pytest)
• Code formatted (ruff format and ruff check)
• Pre-commit hooks pass (pre-commit run --all-files)

Verification performed

• uv run pytest tests/test_http_server_api.py -q passes.
• Manual smoke checks against running server:
  • GET /health returns {"status":"ok"}
  • GET /indexes returns indexed entries
  • GET /openapi.json returns valid schema including /indexes/{index_name}/search
  • POST /indexes/env-check/search with JSON body returns search results