Add five new attributes to the `job` and `job_postprocessing` tracing spans
so that OTEL-consuming backends (Sentry, Honeycomb, Datadog, etc.) can
filter and group telemetry by how a job was triggered and what type it is.
New span attributes:
- `job_kind` — Script, Flow, AppScript, AIAgent, Preview, etc.
- `created_by` — the user or system identity that queued the job
- `trigger_kind` — schedule, webhook, kafka, http, sqs, etc.
- `trigger` — the schedule/trigger path (when applicable)
- `runnable_id` — the id of the runnable that ran
Also adds `JobKind::as_str()` for a consistent lowercase string
representation, following the same pattern as `ScriptLang::as_str()`.
Existing attributes (job_id, workspace_id, script_path, language, tag,
flow_step_id, parent_job, root_job) are unchanged.
Note: the EE `full_job` span in `otel_ee.rs` and the log records emitted
by `job_logger_ee.rs` would also benefit from these attributes. This PR
covers only the public-repo spans; a follow-up EE change would propagate
the same fields to logs and the full_job span.
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