feat: run comparison - run metadata comparison

[camielvs]

Description

Fifth PR in the Compare Runs stack. Adds run-level metadata comparison — the context around the two runs, as opposed to their pipeline structure.

It compares who created each run, when, and any run-level annotations and arguments, and surfaces what changed. Two things are deliberately handled specially:

• Created-at is shown for context but never counts as a "change" on its own — two different runs always have different timestamps.
• Superficial keys (notes, tags) and layout-only editor annotations are ignored so they don't create noise.

Annotation and argument keys are treated generically, so whatever a given deployment stores shows up without needing hardcoded knowledge of the keys. Rendered as a compact summary bar that expands to show the details when something actually differs.

Related Issue and Pull requests

Part of the Compare Runs stack. Builds on #2599; the next PR (#2601) builds on this branch.

Type of Change

• New feature

Checklist

• I have tested this does not break current pipelines / runs functionality
• I have tested the changes on staging

Screenshots (if applicable)

Test Instructions

• Run the unit tests: npm run test -- compareRunMetadata
  • Covers author changes, the created-at exception, generic annotation/argument diffing, and exclusion of superficial/frontend-only keys.
• End-to-end (via the top of the stack, feat: run comparison - compare page, run selection & routing #2603, with the Compare runs flag on):
  • Compare two runs by different authors and confirm the metadata bar flags the author change and expands to show details.
  • Confirm two runs by the same author with only different timestamps are reported as unchanged.

Additional Comments

[camielvs]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: run comparison - compare page, run selection & routing #2603 : 2 dependent PRs (#2604 , #2641 )
• feat: run comparison - run picker & switcher #2634
• feat: run comparison - graph diff view #2602
• feat: run comparison - structured diff view #2601
• feat: run comparison - run metadata comparison #2600 👈 (View in Graphite)
• feat: run comparison - artifact comparison #2599
• feat: run comparison - shared diff UI primitives #2598
• feat: run comparison - merged graph & change summary logic #2597
• feat: run comparison - pipeline diff engine #2596
• master

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[github-actions]

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A preview build has been created at: cmp-05-run-metadata-comparison/fbe96eb