docs: enhance PR template — diagnosis prompt, N/A guidance, verification evidence

[nazirulhafiy]

What does this PR do?

Improves the PR template based on a review of PR #18998 by @helix4u, which set a high bar for documentation quality. Changes are additive — no sections removed.

Changes

• "What does this PR do?" — now prompts for root cause + diagnosis alongside the fix description
• Checklist header — added N/A guidance: check boxes to show consideration, fill every section or explain why
• Code checklist — added "I've noted any pre-existing test failures outside my change" (the honesty move from PR fix(model): avoid Bedrock credential probe in provider picker #18998)
• "Screenshots / Logs" → "Verification Evidence" — renamed and expanded to prompt for test output, before/after logs, profiling traces. Includes a worked example referencing real PR behavior

Related Issue

N/A. Prompted by a documentation review of existing PR patterns.

How to Test

Open the template file and verify the new sections render correctly.

Checklist

• I've read the Contributing Guide
• My PR contains only changes related to this fix
• Template change only — no tests needed
• Tested locally: file renders clean in markdown

Documentation

• This is the documentation update
• N/A — no config changes
• N/A — no workflow changes
• N/A — no cross-platform impact
• N/A — no tool changes

Verification Evidence

Template renders as expected. 14 lines added, 4 removed, 0 sections restructured.