docs(python): align integration READMEs with API v2

[tryambak2019]

Summary

Correct Python integration documentation that described removed or unsupported API behavior.

Implemented

• Remove the deleted REST session surface and obsolete constructor arguments.
• Distinguish in-memory A2A tasks from LangGraph SQLite checkpoints.
• Remove incomplete setup and unverified currency CLI, build, deployment, and Docker instructions.
• Clarify that endpoint values are currently required but unused for outbound connections.

#2689 Series

• Documentation corrections: docs(python): align integration READMEs with API v2 #2821 (this PR)
• Currency entry-point and startup validation: fix(python): make currency CLI entry point importable #2829
• 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Superseded by #2887, which consolidates the requested Python startup and documentation changes for review.