fix(autofix): Reset sections on re-run

[Zylphrex]

When the same step is re-run, the contents from the previous run should be discarded to make for a cleaner loading state.

[sentry]

Bug: The mergedByFile map is not cleared when a step re-runs, causing artifacts from previous runs to be incorrectly included in the output.
_{Severity: MEDIUM}

Suggested Fix

When a step re-run is detected within the getOrderedAutofixSections function, clear the mergedByFile map. This should happen inside the if (metadata.step === section.step) block to ensure each step execution starts with a clean state.

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: static/app/components/events/autofix/useExplorerAutofix.tsx#L379

Potential issue: In the `getOrderedAutofixSections` function, the `mergedByFile` map
accumulates file patches. When a step like `code_changes` is re-run, the logic is
intended to discard the previous run's results. However, while the current `section` is
reset, the `mergedByFile` map is not cleared. This causes file patches from the previous
run to be carried over. If the re-run generates new patches, the final artifacts will
incorrectly contain both the old and new patches, rather than just the results from the
most recent run.

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