Clear session-scoped approval state on /resume and /branch

[Es1la]

Summary

This fixes a session-boundary state leak in the gateway.

Before this change, session-scoped dangerous-command approvals and /yolo
state were keyed by session_key, but they were not cleared when the user
switched conversations via /resume or /branch. That meant approval state
from the previous conversation could silently carry over into the newly
resumed or branched conversation.

What changed

• added GatewayRunner._clear_session_boundary_security_state(session_key)
• clear session-scoped approval state after a successful /resume switch
• clear session-scoped approval state after a successful /branch switch
• clear any stale gateway _pending_approvals entry for that session as part
  of the same boundary cleanup

Why this is correct

/resume and /branch are real conversation-boundary operations:
they switch the active transcript to a different session. Session-scoped
dangerous-command approvals should not survive that boundary.

This matches the existing isolation direction already applied to /new and
other session-boundary cleanup paths.

Tests

Added regression tests covering:

• /resume clears session-scoped approval state
• /resume clears session-scoped /yolo state
• /resume clears only the active session’s pending approval cache
• /branch clears session-scoped approval state
• /branch clears session-scoped /yolo state
• /branch does not affect other sessions

Validated with:

uv run pytest tests\gateway\test_session_boundary_security_state.py tests\gateway\test_resume_command.py tests\gateway\test_yolo_command.py -q -n 4

[alt-glitch]

Related to #10702 (gateway /resume leaking cached agent state) and #10665 (clear approval state on /new and /reset). This PR extends the same cleanup pattern to /resume and /branch.

[teknium1]

Merged via PR #14266 with your original commit preserved (rebase merge — authorship intact in git log). Also extended the same fix to /new in a follow-up commit on top. Thanks @Es1la!