Add test: verify coldbrew.trace_id OTEL span attribute in SetTraceId

[ankurs]

Context

PR #22 added coldbrew.trace_id as an OTEL span attribute in SetTraceId() (notifier/notifier.go). This links the ColdBrew correlation ID to distributed traces for cross-system debugging.

The attribute is set in two paths:

1. Early return — when trace ID already exists, sets attribute before returning
2. Normal path — after resolving trace ID from metadata/OTEL/UUID, sets attribute on the span

What's needed

Add a unit test using the OTEL SDK test tracer (go.opentelemetry.io/otel/sdk/trace/tracetest) to verify:

• coldbrew.trace_id attribute is set on the span when SetTraceId is called with an active OTEL span
• Metadata-supplied trace ID takes priority over OTEL-generated trace ID
• When trace ID already exists (early return path), the attribute is still set
• When no span is in context, no panic occurs

Files

• errors/notifier/notifier.go — SetTraceId() function (lines 536-573)
• errors/notifier/notifier_test.go — add tests here