fix(telemetry): keep participant identity out of log message bodies

[omChauhanDev]

Problem

Redaction here is field-level filtering of log attributes.
telemetry.pii._PIIFilteringLogProcessor.on_emit rewrites record.attributes and
_TraceLevelLoggingHandler._translate does the same; neither touches the record body. The comment
in traces.py says why:

callers pass user data through extra={"lk.pii.<name>": ...} precisely because a log body
cannot be redacted

Four call sites interpolate a participant identity or a room name into the message body, so those
values are exported as written even when the project has redaction enabled:

• job.py, the duplicate participant-task warning and the participant-entrypoint error
• beta/workflows/warm_transfer.py, the caller-disconnect log and the room-move log

Both categories are the ones REVIEW.md lists first, and both already have a marked constant in
telemetry/trace_types.py: ATTR_PARTICIPANT_IDENTITY and ATTR_ROOM_NAME.

Changes

Each message becomes static and the value moves to extra, with the key written as a
lk.pii.<name> literal, which is how every other logger extra in the tree writes it
(llm/utils.py, beta/workflows/dtmf_inputs.py, voice/agent_activity.py). No new imports, and
job.py's warning no longer needs its # noqa: E501.

exc_info on the entrypoint error is unchanged: the exception message and stacktrace are already
handled by _TraceLevelLoggingHandler.

Both files are in one change because the new test is tree-wide, so fixing only one leaves it red.

Testing

tests/test_telemetry_pii_log_bodies.py parses livekit-agents/ and livekit-plugins/ and fails
when a plain name or attribute chain matching an identity or a room name is interpolated into a
logger message. On main it reports all five expressions across the four sites. It reads
expressions rather than values, so it stays narrow: f"{p.identity}" is flagged,
f"{identity is not None}" is not.

pytest --unit --audio_eot: 2575 passed, 5 skipped, against 2574 on unmodified main.
ruff check and ruff format --check (995 files) clean, scripts/check_types.py reports no
issues in 650 source files.
All at 4d03f505c.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.