ci(e2e): refuse a focused batch whose concurrency group exceeds 400 characters

[teamleaderleo]

Why

test-e2e.yml builds its concurrency group from the runner, the ref and the whole test_filter:

e2e-<runner>-<40-hex ref>-<test_filter>

A batch of four long Swift Testing selectors produced a 520-character group, and run 35939770225 was created with no jobs. GitHub reported only "This run likely failed because of a workflow file issue". run-e2e.sh printed the run URL as if the dispatch had worked.

GitHub does not document the limit. Recent dispatches show where it falls:

group length (approx.)	run	jobs
372	35930869627	4
393	35935925123	3
437	35938283242	0
462	35930690423	0
520	35939770225	0

So the cutoff is between 393 and 437. This PR uses 400.

Change

dispatch-focused-test.py computes the group the workflow will build and refuses the dispatch before any runner is spent. The error names the length and says to split the selectors or select the whole suite. When the pool can't be resolved, it measures against the longest known runner label.

Verification

• A new launcher test (902047d is the test and 11c1768 the fix): four long selectors are refused and nothing is dispatched; two of the same selectors still dispatch as one batch.
• tests/test_run_e2e.py: 52 pass.
• All 123 guard commands listed in ci-guards.yml pass locally.

🤖 Generated with Claude Code

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Summary by cubic

Refuses focused test dispatches whose concurrency group would exceed 400 characters, preventing the silent broken runs GitHub creates for oversized groups.

• Previously such a batch dispatched and started with zero jobs and no explanation; now dispatch-focused-test.py measures the group length first and errors with a hint to split selectors or run the whole suite.
• When the runner pool can't be resolved, the length check uses the longest known runner label.
• The launcher test uses three selectors whose filter alone is under 400 but whose whole group exceeds it, and pins the workflow's group expression so the dispatcher's copy cannot drift silently.

^{Written for commit 5985754. Summary will update on new commits.}

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📥 Commits

Reviewing files that changed from the base of the PR and between 229edc5 and 5985754.

📒 Files selected for processing (2)

• scripts/ci/dispatch-focused-test.py
• tests/test_run_e2e.py

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[teamleaderleo]

An independent review subagent found nothing blocking at 11c1768. It confirmed:

• the computed group matches test-e2e.yml on every path the dispatcher sends (explicit, auto, routed large runner, unknown default);
• the check runs before any dispatch or history lookup, and --force does not skip it;
• the test fails without the fix.

It also found two more zero-job runs that narrow the limit: 35834658722 (group 407) and 35830735643 (group 426). The cutoff is therefore between 394 and 407.

Pushed 5985754 for its low-severity points:

• The refused case now uses three selectors. The filter is 377 characters and the group is 448, so a check on the filter alone would now fail the test. Before, anything between 324 and 575 passed.
• The test asserts the workflow's concurrency.group expression verbatim, so the dispatcher's copy of it cannot drift silently.
• The unknown-runner fallback no longer calls default_runner() a second time, which could add a 30 s timeout.

tests/test_run_e2e.py: 52 pass.

— Ibex g1 🌿