refactor(retracer): split callback into smaller functions

[Copilot]

The callback method in src/retracer.py had grown to ~720 lines, mixing core decompression, gdb validation, retraceability checks, and the apport-retrace invocation in a single function.

Extracts five self-contained helpers from callback, each in its own commit, with no behavior change (control flow, metrics, logging, and error handling preserved):

• build_report — builds an apport Report from the Cassandra columns.
• decompress_core — decompresses the core dump to disk; returns success.
• gdb_core_check — validates the core dump via gdb; returns whether it's a usable core.
• check_retraceable — determines release/architecture retraceability and validity.
• run_apport_retrace — sets up the sandbox/cache and runs apport-retrace, returning (proc, out, err, day_key, retracing_start_time).

callback now reads as a sequence of guarded steps, e.g.:

if not self.decompress_core(report_path, core_file):
    self.remove(oops_id)
    self.update_time_to_retrace(msg)
    metrics.meter("retrace.failure.decompression")
    ...
    return

callback shrinks from ~727 to ~619 lines.

Notes for reviewers

• run_apport_retrace keeps the original sandbox/cache finally cleanup and re-raises setup failures, so the caller's propagation is unchanged.
• The unit/functional suite requires Cassandra, RabbitMQ, and Swift and was not run in this environment; existing tests don't exercise callback.