feat: run comparison - run picker & switcher

[camielvs]

Description

The Compare Runs stack needs a way to actually choose the two runs you're comparing. This PR adds that: the run chips in the page header, and the dialog behind them.

• Run chips (A and B) — each side of the comparison is a small coloured chip: blue for A, green for B. The chip shows the pipeline name and run number, and gives you three things: click the name to open that run in a new tab, a swap-in button to replace it with a different run, and an ✕ to clear the slot. An empty slot is a dashed "Select run A / B" button instead.
• Run picker dialog — opens from either chip. It's a searchable list of recent runs with:
  • search by pipeline name (debounced, with a clear button),
  • a created by filter,
  • a date range filter (UTC),
  • one row per run showing status icon, pipeline name, run number, the task status bar, when it ran and who ran it.
• The run already selected on the other side is filtered out of the list, so you can't accidentally compare a run with itself.

Two small things worth knowing:

• The picker reuses the same cached run list as the runs dashboard, so opening it with no filters applied is instant rather than a fresh fetch.
• Nothing in this PR is rendered yet — these are the pieces the compare page in feat: run comparison - compare page, run selection & routing #2603 mounts. Reviewing this on its own is a component read; to see it on screen, use feat: run comparison - compare page, run selection & routing #2603.

Related Issue and Pull requests

Part of the Compare Runs stack. Builds on #2602, consumed by #2603 (#2596 → #2603).

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

These components are only visible once #2603 is in, so test from the tip of the stack:

1. Enable the Compare runs flag under Settings → Beta features and open the compare page.
2. Click Select run A — the picker opens. Try each filter: type a pipeline name, pick a created by value, set a date range. Confirm the list narrows and shows "No runs match your filters." when nothing matches.
3. Pick a run — the dialog closes and the chip fills in with the pipeline name and run number.
4. Repeat for B, and confirm the run you picked for A is not in B's list.
5. On a filled chip: click the name (opens that run in a new tab), the swap-in icon (reopens the picker), and ✕ (clears the slot).
6. Confirm the runs dashboard still lists and filters runs as before — the picker shares its query cache.

Additional Comments

No existing behaviour changes; everything here is new and scoped to the compare page.

[github-actions]

🎩 Preview

A preview build has been created at: cmp-08-run-picker/7766fb7

[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 👈 (View in Graphite)
• feat: run comparison - graph diff view #2602
• feat: run comparison - structured diff view #2601
• feat: run comparison - run metadata comparison #2600
• 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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