OTel: log correlation — trace_id/span_id on JSON log records

[FarhanAliRaza]

Follow-up to #6227 (ENG-9166), split out of the reflex-otel stack (#6899 → #6900 → #6901) because it depends on the logging pipeline (#6863).

What

When OpenTelemetry tracing is enabled (reflex_base.otel.enabled), every framework log record emitted inside an active span should carry the W3C ids so a log line can be looked up in the tracing UI and vice-versa:

• JsonHandler.emit (reflex_base/utils/log.py) adds trace_id (32 hex) and span_id (16 hex) to the payload when trace.get_current_span().get_span_context().is_valid.
• Add reflex_otel to PACKAGE_LOGGER_NAMES so its records join the reflex logger hierarchy.
• Optional: the same fields on the rich console/file formatter in debug mode.

The helper existed as otel.trace_fields() in an earlier revision of #6900 (removed when decoupling); the JsonHandler hunk is ~6 lines plus a test in tests/units/reflex_base/utils/test_log.py (test_json_mode_adds_trace_ids).

Blocked by

• ENG-10963 feat(log): add stdlib logging pipeline in reflex_base.utils.log (1/5) #6863 (JsonHandler) — and ENG-9166 feat(otel): built-in OpenTelemetry trace points, metrics and the reflex-otel instrumentor (1/3) #6899 (reflex_base.otel).

Docs

Add a "Logs" section to docs/api-reference/observability.md: with REFLEX_LOG_JSON=true, records inside a span carry trace_id/span_id; user loggers can use opentelemetry-instrumentation-logging.

[linear-code]

ENG-11327