fix(python): make basic OpenAI agent entry point importable

[tryambak2019]

Summary

Restore the published basic OpenAI agent package entry point and validate its limited startup path.

Implemented

• Add the basic-openai-agent console entry point.
• Extract build_app() while preserving the package's public app export.
• Run the container through the installed entry point so package imports resolve.
• Add CI coverage that invokes main() with Uvicorn mocked and verifies GET /health.

Validation

• OPENAI_API_KEY=fake make -C python test: 552 passed, 1 skipped.
• uv lock --check, Ruff lint, and Ruff format checks passed.
• Basic OpenAI agent image builds successfully.

#2689 Series

• Documentation corrections: docs(python): align integration READMEs with API v2 #2821
• Currency entry-point and startup validation: fix(python): make currency CLI entry point importable #2829
• Basic OpenAI agent entry-point and startup validation: this PR
• Remaining LangGraph execution/deployment validation, ADK and AgentSTS audits, CrewAI validation, and residual cleanup: Revisit Python packages and samples after API v2 cutover #2689

Related to #2689. This PR does not close it.

[tryambak2019]

Superseded by #2887, which consolidates the requested Python startup and documentation changes for review.