docs: document client request context

[MukundaKatta]

Summary

• document per-request MCP metadata with the high-level Client API
• clarify that HTTP headers/auth/timeouts belong on a configured httpx.AsyncClient passed to streamable_http_client

Fixes #1966

Testing

• git diff --check
• python3 assertion that README context docs are present

[maxisbey]

Closing — README.md is frozen at v1 on main (a pre-commit hook rejects edits). Doc changes for v2 go in README.v2.md. See AGENTS.md for the branching model. Thanks for the contribution.

_{AI Disclaimer}