fix(CI): flaky tests and retry merge queue tests with backoff

[dayaffe]

Description

Adds retries to the merge queue test runs and fixes the most common flaky tests from the last two months of CI.

Retries. Unit tests, make test-all and coverage now use the nextest ci profile:

• Failing tests are retried twice, with a 1–4s backoff. Retries used to run back to back without backoff which didn't help.
• The run continues after a failure. make test-all used to stop at the first one. This means if multiple tests fail in make test-all all the failures will be reported instead of just one
• A single test running for more than 10 minutes is killed and retried, instead of running until the job times out.

Flaky test fixes. All test-side; mostly tests not waiting long enough or waiting on the wrong thing.

Test	Cause	CI jobs affected (Aug–Sep)
Kafka tests	used a topic before Kafka finished creating it (the 09-30 failure)	13
test_ingest_v1_happy_path	ingested into a new index before it was ready, got a 404	13
test_ingester_close_shards	held a lock the code under test also needed	4
retiring-indexer / graceful-shutdown tests	exact doc counts fail if a document is written twice, which ingest v2 allows	2
test_indexer_exceeding_max_num_partitions	the test's fast-forwarded clock could skip past a timeout	1

How was this PR tested?

make test-all passes, and clippy and fmt are clean.

Each fixed test was run repeatedly on an overloaded machine with retries off, before and after the fix:

Tests	Failed runs before	Failed runs after
Kafka tests	2 of 30	0 of 30
test_ingester_close_shards	2 of 300	0 of 300
test_indexer_exceeding_max_num_partitions	2 of 200	0 of 200
Retiring-indexer and graceful-shutdown tests	4 of 20	0 of 120
test_retiring_indexer_receives_empty_plan crash	1 in 3 full-suite runs	0 of 60
Janitor delete and garbage-collection tests	5 of 200	0 of 200
Health-check tests (heavier load)	21 of 30	0 of 30

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-07T18:00:43.877797Z	b7c652b	New commits

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[pront]

This is good improvement that will reduce a lot of the flakey runs. One followup item worth looking into is if we can refactor tests and remove the need for timeouts.

[guilload]

Isn't this test here to test that we keep track of backpressure and so increasing the queue capacity and asserting that backpressure = 0 defeats its entire purpose?

[dayaffe]

I re-read the test and you're correct, reverting this change.