feat(client): add explicit session_id support for Streamable HTTP resumption

[mrutunjay-kinagi]

Summary

Adds first-class support for reusing a previously issued mcp-session-id on the Python client side.

Note: this follows the same high-level design as the C# implementation for session resumption in modelcontextprotocol/csharp-sdk (#946, implemented by #1029):

• transport-level known session IDs
• explicit client resume flow that skips initialize
• caller control over session ownership/termination behavior

Closes #2116

Changes

• streamable_http_client(...) accepts session_id: str | None and seeds transport state with it.
• StreamableHTTPTransport accepts optional initial session_id.
• URL Client now supports:
  • streamable_http_session_id
  • streamable_http_terminate_on_close
  • streamable_http_initialize_result
• Added Client.resume_session(...) explicit resume constructor.
• Added ClientSession.resume(...) to apply previously negotiated initialization data and skip the initialize handshake.
• streamable_http_client(...) now starts GET stream immediately when resuming with a known session ID.
• StreamableHttpParameters supports session_id and forwards it in ClientSessionGroup.

Validation

• .venv/bin/pytest -q tests/client/test_client.py -k "streamable_http or resume_session or requires_session_id"
• .venv/bin/pytest -q tests/client/test_session_group.py -k "establish_session_parameterized"
• .venv/bin/pytest -q tests/shared/test_streamable_http.py -k "seeded_session_id_header"

[mrutunjay-kinagi]

Follow-up on latest CI for commit dbddc8b:

• Workflow run succeeded: https://github.com/modelcontextprotocol/python-sdk/actions/runs/23387443373\n- Required gate all-green passed.
• Four 3.11 matrix jobs are shown as failed individually, but these are non-blocking in this workflow configuration and are aggregated by all-green.