Raise test coverage above 92% in adapters, span_factory, and webhook processor

[NewerKey]

Current: 148 tests passing, 92% overall coverage
(`uv run pytest --cov=src --cov-report=term-missing`). Not urgent —
mostly error-handling branches — but tracked so it doesn't quietly drift
lower as new adapters (#1) land.

Weakest spots as of 2026-07-11:

• `adapters/gitlab.py` — 85% (lines 62-63, 77, 149-151, 166-167,
  182-183, 211-213, 233-234, 267-269, 278-280, 289, 296-297)
• `adapters/github.py` — 86% (lines 147-149, 156-157, 176-178, 221-223,
  238-239, 270-272, 296-298)
• `webhooks/processor.py` — 86% (lines 50, 80, 110, 142, 172-173,
  214-217)
• `span_factory.py` — 88% (lines 31, 58-59, 99-100, 104, 114-115,
  129-131)
• `server/routes.py` — 88% (lines 36, 61-62, 65)
• `payload_sink.py` — 89% (lines 58-60)
• `telemetry.py` — 92% (lines 74-75)
• `cli.py` — 94% (line 39)