fix(python): consolidate API v2 startup validation

[tryambak2019]

Summary

Consolidates the related Python startup and documentation work requested in the review of #2832.

• Corrects retained package and sample documentation to remove obsolete Kagent-owned session, controller, and persistence claims.
• Restores installed sample entry points, Docker commands, ASGI construction, and GET /health coverage for currency, OpenAI, LangGraph, ADK, and CrewAI samples.
• Runs the combined sample suite with import-safe test collection and prevents reintroduction of removed Kagent REST session APIs.

Supersedes #2821, #2829, #2830, #2831, #2832, and #2834. #2833 is already merged and is not included.

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

Validation

• OPENAI_API_KEY=fake make -C python test: 586 passed, 1 skipped (Python 3.13)
• Python test matrix completed on 3.11, 3.12, and 3.13
• uv lock --check, Ruff format/lint, and git diff --check
• Built and health-probed the seven retained sample images with placeholder runtime configuration.

Live A2A requests, Kubernetes/BYO deployment, and durable checkpoint/restart behavior remain unvalidated and are explicitly not claimed by this PR.

[tryambak2019]

@supreme-gg-gg This is the combined replacement requested in #2832. It supersedes #2821, #2829-2832, and #2834; combined tests and sample health checks are included. Could you please review #2887?