fix(python): make currency CLI entry point importable

[tryambak2019]

Summary

Restore the published currency package entry point and its container command.

Implemented

• Replace the invalid absolute import with from .agent import graph.
• Run the container through the installed currency console entry point.
• Extract build_app() so main() and the test use the same ASGI construction path.
• Add CI coverage that calls main() with Uvicorn mocked and verifies GET /health.

Validation

• Local Python suite: 552 passed, 1 skipped.
• uv lock --check and Ruff checks passed.
• Currency 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 (this PR)
• Remaining package/sample inventory, A2A execution, deployment, and checkpoint-restart validation: 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Superseded by #2887, which consolidates the requested Python startup and documentation changes for review.