[iFixRobots]

I noticed that BeeperTerseJsonFormatter drops the useful part of an exception. Callers pass exc_info=(type, value, traceback), but the shared JSON formatter only emits exc_type and exc_value. A KeyError therefore tells us which key was missing without showing the file, function, or line that tried to read it.

This adds an exc_traceback field to the Beeper formatter using Python's existing formatException. The original fields and request metadata stay intact, and traceback newlines are JSON-escaped so each record remains one physical log line. Ordinary records are unchanged. The upstream JSON formatters and the GCP formatter are left alone.

I exercised the actual formatter with an ordinary record, a chained KeyError/RuntimeError, and exc_info=True without an active exception. The chained traceback survived JSON decoding, all three records remained single-line JSON, and local-variable values were not captured. The existing Trial formatter suite could not start locally because Twisted is not installed.

The first CI image build failed before building Python: the workflow used the branch name verbatim as a Docker tag, and / is not valid in a tag. I also normalized branch-name slashes to - in that step; release-version tags are unchanged.

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PR Summary

Beeper's JSON log formatter previously kept only the exception type and message, so logged errors lost their file/line context. This PR adds the formatted traceback to those records and fixes CI image builds for branch names containing /.

• BeeperTerseJsonFormatter adds an exc_traceback field (from Formatter.formatException, chained exceptions included) whenever a record carries a real exception. exc_type, exc_value, and extras are unchanged. Records stay single-line JSON.
• Upstream JsonFormatter/TerseJsonFormatter and GcpJsonFormatter are untouched.
• beeper-ci.yaml replaces / with - in branch-derived Docker tags. Release tags on beeper are unchanged.

Issues

1 potential issue found:

• No test in tests/logging/test_terse_json.py covers BeeperTerseJsonFormatter or the new exc_traceback field, so a later upstream merge could drop Beeper tracebacks without anyone noticing. Add a small exception test next to the existing JsonFormatter one.

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⚪️ Nit · No test for the new exc_traceback field

JsonFormatter and GcpJsonFormatter both have exception tests (test_with_exception, test_gcp_json_with_exception), but nothing tests BeeperTerseJsonFormatter. I ran the formatter by hand and it works: chained traceback present, one line per record, nothing emitted for exc_info=True with no active exception. A test would keep that behavior from regressing when upstream changes to JsonFormatter._format get merged in.