Add Block Kit layout to frontend-triage notifications

[suhaibmujahid]

Resolves #6673

Refactors notification rendering into reusable helpers and adds build_blocks for richer Slack messages.

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[Copilot]

Pull request overview

This pull request implements Slack Block Kit formatting for the frontend-triage agent’s auto-apply notifications, improving readability while preserving the existing plain-text message as Slack’s fallback.

Changes:

• Refactors notification rendering into small helpers (_bug_link, _summary, _is_urgent, field renderers).
• Adds build_blocks(...) to generate a Block Kit layout (headline, optional fields grid, run context).
• Updates notification recording and tests to ensure blocks is included and that Block Kit conveys at least the same facts/URLs as the fallback text.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agents/frontend-triage/hackbot_agents/frontend_triage/notify.py	Adds Block Kit block building and records Slack actions with both text and blocks.
agents/frontend-triage/tests/test_notify.py	Extends tests to validate Block Kit structure and equivalence of conveyed facts vs fallback text.

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[crythms]

@suhaibmujahid
#6679 and this PR conflict. I think #6679 would need to land first. it targets master directly while this sits on top of other unmerged PRs (#6674 → #6675 → #6676), and it changes whether the severity in these notifications was actually
applied to the bug.

#6679 removes the triage agent's ability to write severity to Bugzilla at all. Severity just becomes a suggestion in the triage comment, and is left out below medium confidence.

After #6679 lands, this would need a rebase plus a couple of changes:

1. _severity_field adds the word "suggested" only when confidence is below high. That's right today, because a high-confidence severity really does get written to the bug. After Frontend triage severity as comment suggestion #6679 nothing is ever written, so a plain S3 would look like the bug was changed when it wasn't. The if could go and the label could say it instead -- *Suggested severity* with the level underneath, every time. The build_blocks headline has the same problem: it shows a bare *S1*.

2. _is_urgent and _severity_field both ignore confidence. Frontend triage severity as comment suggestion #6679 only reports severity at high or medium (REPORTABLE_SEVERITY_CONFIDENCES in config.py), so these would need to match, otherwise Slack flags an S1 on a bug whose comment doesn't mention severity at all.

A few tests in this PR assume the old behaviour too, but those should fail loudly on the rebase so they're easy to spot.