fix: fail fast when session is not started

[Epochex]

When ClientSession/BaseSession is used without entering the async context manager, the receive loop never starts and initialize() can hang indefinitely waiting for a response.

This change fails fast: send_request() and send_notification() now raise a RuntimeError if the session hasn't been started via async with ... as session.

Fixes #1452.

Tests:

• uv run --frozen python -m ruff check src/mcp/shared/session.py tests/client/test_session.py
• uv run --frozen python -m ruff format src/mcp/shared/session.py tests/client/test_session.py
• uv run --frozen pytest tests/client/test_session.py -q

[Epochex]

Follow-up for CI: Python 3.14 was still failing branch coverage due to a missing arc reported on the pytest.raises(...) context manager line in ests/client/test_resource_cleanup.py. Added # pragma: no branch there to make branch coverage stable across 3.14. Pushed commit abbbc01.