refactor: share error taxonomy between metrics and tracing plugins

[ajbozarth]

Background

Retired sub-issue #475 proposed creating mellea/telemetry/errors.py. That's no longer needed — mellea/telemetry/metrics_plugins.py already has classify_error() and 8 ERROR_TYPE_* constants, built during the metrics epic (issue #465).

Scope

• Extract classify_error and ERROR_TYPE_* constants into a shared location (e.g., mellea/telemetry/errors.py or mellea/telemetry/_shared.py) so both metrics and tracing plugins can import from one source.
• Have tracing plugins set mellea.error.type (semantic category) and error.type (exception class name) on error spans, matching metrics' labels. feat(telemetry): close five OTel GenAI semantic convention emission gaps (#1035) #1036 already sets error.type on the error path; this issue adds the semantic category alongside it.

Phase & dependencies

Phase 3 (polish). Depends on #1045 and #1047. Can start any time after Phase 1 lands.

Acceptance criteria

• One source of truth for error classification across metrics and tracing.
• No duplicate ERROR_TYPE_* constants.
• Error spans carry both mellea.error.type (semantic) and error.type (exception class).

Parent epic: #444