chore(tests): disk_v2 antithesis test harness + exercisers

[blt]

Summary

The Antithesis test harness for disk buffer v2: two self-driving exercisers
(disk_v2_antithesis — the #21683 accounting cluster; disk_v2_lossfinder — a
7-scenario data-loss fault menu with a no-silent-loss oracle), plus Dockerfiles,
docker-compose configs (block / drop variants / direct / lossfinder), the Vector-
SUT workload, and the test templates under tests/antithesis/test/v1. Registers
the two examples + a serde_json dev-dep.

Stack (Antithesis-driven disk buffer v2 data-loss investigation):

• chore(tests): disk_v2 data-loss research scratchbook #25524 — research scratchbook (docs)
• chore(unit tests): failing disk_v2 bug demonstrations #25525 — failing bug-demonstration tests
• feat(buffers): SUT-side antithesis underflow assertions #25526 — SUT-side assertions
• chore(tests): disk_v2 antithesis test harness + exercisers #25527 — test harness + exercisers

How did you test this PR?

cargo build -p vector-buffers --example disk_v2_antithesis --example disk_v2_lossfinder;
snouty validate on both config dirs (reaches setup_complete, test commands
discovered); local smoke runs of both exercisers. Antithesis runs reproduced #21683
and the finalizer status-discard loss.

Change Type

• Bug fix
• New feature — test infrastructure (no runtime behavior change)
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: buffer_byte_size has value 18446744073709548000 #21683, vector_component_discarded_events_total metric not showing/incrementing for gcp_pubsub sink when disk buffer is full (drop_newest) #24606, Sinks configured with disk buffers cause partial stall of pipeline on config reload #24948

[blt]

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(buffers): SUT-side antithesis underflow assertions #25526
• chore(tests): disk_v2 antithesis test harness + exercisers #25527 👈 (View in Graphite)
• chore(tests): disk_v2 data-loss research scratchbook #25524 : 1 other dependent PR (#25525 )
• master

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[datadog-vectordotdev]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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