test(db): stabilize delayed checkpoint

[corylanou]

Description

Rework the delayed-checkpoint regression test so it controls checkpoint eligibility explicitly and waits for TXID advancement with a bounded condition poll.

The test now verifies the write sync advances replication before enabling a later time-based checkpoint. Production checkpoint behavior is unchanged.

Motivation and Context

The failed CI attempt showed the initial checkpoint's post-processing outliving the test's 100 ms interval. The supposedly immediate write sync therefore performed the checkpoint early (insert=4), leaving the later sync at the same TXID (delayed=4). The retry passed, confirming the runner-speed dependency.

In scope: deterministic checkpoint setup, eligibility control, bounded polling, and diagnostic timeout output.

Out of scope: production checkpoint logic and unrelated concurrent-write test logging.

Fixes #1495

How Has This Been Tested?

go build ./...
go test ./...
go test -race ./...
go test -race . -run '^TestDB_DelayedCheckpointAfterWrite$' -count=30
pre-commit run --all-files
golangci-lint run --new-from-rev=origin/main

The original failure was also reproduced deterministically before the rewrite, yielding insert=4 delayed=4.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[github-actions]

PR Build Metrics

✅ All clear — no issues detected

Check	Status	Summary
Binary size	✅	37.18 MB (0.0 KB / 0.00%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.14 (latest)
Module graph	✅	1230 edges (0)

Binary Size

	Size	Change
Base (4ed7a30)	37.18 MB
PR (bf71f78)	37.18 MB	0.0 KB (0.00%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 1 vulnerability in packages you import and 2
vulnerabilities in modules you require, but your code doesn't appear to call
these vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	45s
Go version	go1.25.14
Commit	bf71f78

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_{🤖 Updated on each push.}