feat(autofix): Disable rethink button when textarea empty and allow submit on Enter

[aliu39]

Improves the rethink textarea UX in the autofix next-step flow:

• The "Rethink" submit button is now disabled when the textarea is empty or whitespace-only, preventing no-op submissions.
• Pressing Enter (without Shift) while the textarea is focused submits the rethink, matching chat-style submit conventions. Shift+Enter still inserts a newline.

[cursor]

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

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit b1c2adc. Configure here.}

[sentry]

Sentry Snapshot Testing

Name	Added	Removed	Modified	Renamed	Unchanged	Status
sentry-frontend sentry-frontend	0	0	0	0	204	✅ Unchanged

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

---

"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.44%

[sentry]

Bug: The "Re-run from here" button in AutofixResetPrompt is disabled by default, preventing users from re-running a step without providing new context, which is a valid action.
_{Severity: MEDIUM}

Suggested Fix

The "Re-run from here" button should be enabled even when the user context input is empty. Remove the disabled={!userContext.trim()} check from the button to allow re-running without providing additional context, which is consistent with the handleReset function's optional parameter.

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/v3/autofixResetPrompt.tsx#L50

Potential issue: In `AutofixResetPrompt`, the `userContext` state is initialized as an
empty string. The "Re-run from here" button's `disabled` attribute is tied to
`!userContext.trim()`, which makes the button disabled by default when the prompt opens.
This prevents users from re-running an autofix step without providing additional
context, a use case that is supported by the `handleReset` function which accepts an
optional context. Once the reset prompt is open, the card-level refresh button is also
disabled, trapping the user in a state where they must either type something or close
the prompt to proceed.

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