chore(llm-detection): Store trace id, category, and subcategory in issue metadata

[nora-shap]

This feels hacky, but I'm wondering if it's possible to include these additional fields on the issue, without displaying them on the frontend, just to make issue quality analysis queries easier.

Summary

• Adds category, subcategory, and trace_id to the event metadata for LLM-detected issues
• These fields flow through the existing ingestion pipeline and persist to sentry_groupedmessage.data["metadata"]
• Enables querying for an llm-detected issue by trace_id
• category, subcategory can now be accessed from the db without having to cross-reference logs

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sentry]

Bug: The metadata dictionary from the LLM detection task is overwritten in the occurrence consumer, losing the trace_id, category, and subcategory fields.
_{Severity: HIGH}

Suggested Fix

In occurrence_consumer.py, modify the logic to merge the existing metadata with the new title field instead of overwriting it. Retrieve the existing metadata from the event payload and update it with the title, rather than creating a new dictionary.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/sentry/tasks/llm_issue_detection/detection.py#L193-L197

Potential issue: In `detection.py`, an event is created with a `metadata` dictionary
containing `trace_id`, `category`, and `subcategory`. This event is sent via Kafka to
the occurrence consumer. However, in `occurrence_consumer.py`, the `_get_kwargs`
function unconditionally overwrites the `metadata` field with a new dictionary
containing only the `issue_title`. As a result, the original metadata fields
(`trace_id`, `category`, `subcategory`) are lost when the event is saved. This prevents
the ability to query for LLM-detected issues by `trace_id` or access the other metadata
fields from the database.

_{Did we get this right? 👍 / 👎 to inform future reviews.}