fix: log exception when output guardrail raises instead of silently ignoring

[cty-ut]

Summary

In RealtimeSession._run_output_guardrails(), exceptions raised by individual
guardrails were silently swallowed with a bare except Exception: continue.
If a guardrail has a bug or misconfiguration, it fails invisibly with no log
output and no way to diagnose the issue.

Fix: add logger.exception(...) before continue so the error is surfaced
in logs while still allowing the remaining guardrails to run.

Test plan

Existing realtime tests continue to pass (make tests). The change is a
one-line addition of a log call in an existing exception handler.

Issue number

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass