[DOCS-13939] Draft AI failure summaries section on browser test results

[git-thuerk-done]

What does this PR do? What is the motivation?

Fixes DOCS-13939

Adds a draft ### AI failure summaries subsection under ## Failed results on the browser test results page. The section describes:

• The contents of a summary (short explanation, true-failure vs test-misconfiguration classification, suggested next steps)
• Where the summary appears (test run details for any failing browser test run)
• A caveat that LLM-generated output can contain inaccuracies and should be used as a starting point, not authoritative RCA
• How to provide feedback on a summary

The section is wrapped in an HTML comment flagging it as a draft. It should not be merged until the feature is publicly released.

Still open

Left as follow-ups because they need either binary assets I don't have or product confirmation:

• Test Details Page screenshots for the 4 pages listed on DOCS-13939 still need refreshed image binaries dropped in at their existing static paths (no markdown changes required):
  • `static/images/synthetics/browser_tests/history.png`
  • `static/images/synthetics/mobile_app_testing/history.png` (and likely `test_details_2.png` — to confirm with reporter)
  • `static/images/synthetics/apm/synthetics-apm.mp4` (video)
  • `static/images/synthetics/guide/synthetics_test_retries/fast_retries.png`
• AI failure summary screenshot for the new section at `static/images/synthetics/browser_tests/test_results/ai_failure_summary.png` (marked with a TODO comment in the markdown)
• Feedback-button label needs product confirmation; current copy says "feedback button" as a placeholder
• Confirm whether the new section stays in this PR or moves to a separate ticket tied to the feature's public release

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

Draft PR — do not merge until feature is publicly available.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/alicia.thuerk/docs-13939-synthetics-ai-failure-summaries/synthetics/browser_tests/test_results
• https://docs-staging.datadoghq.com/alicia.thuerk/docs-13939-synthetics-ai-failure-summaries/synthetics/guide/synthetic-test-retries-monitor-status
• https://docs-staging.datadoghq.com/alicia.thuerk/docs-13939-synthetics-ai-failure-summaries/synthetics/mobile_app_testing/mobile_app_tests/results