test(callbacks): make transient heartbeat-busy lease fixture deterministic

[dennisonbertram]

Work type

Bug

Problem

TestCallbackManagerTransientHeartbeatBusyRetainsClaim races a 90 ms initial lease, a 22.5 ms heartbeat ticker, an injected first renewal failure, and a raw 150 ms sleep. Under race CI the initial lease can expire before the post-busy renewal; production safely releases/retries and starts attempt 2, but the fixture incorrectly expects attempt 1 to remain active.

Evidence: #1143 race CI logged a terminalized started callback at attempt 2 with callback admission unavailable. Current main 4436a590 and #1143 have byte-identical callback production code and the entire failing test. Local exact-main and exact-PR stress passed 100/100, confirming a latent baseline timing race.

Scope

Test/docs only. Instrument transientLeaseStore with deterministic channels for injected first failure and first successful delegated ExtendLease; remove the raw sleep. Use a one-second lease and bounded causal waits.

Out of scope

• Callback product retry/lease semantics.

Acceptance criteria

• Fixture waits for injected failure then first successful renewal, and proves durable renewed deadline exceeds the captured initial deadline while attempt remains 1.
• Starter release/cleanup cannot leave blocked goroutines before manager shutdown.
• No raw timing sleep is used to establish renewal.
• Focused normal/race stress, full regression, and hosted fast/race pass.

Impact/rollback

Test/docs only; no runtime/API/persistence change.

Test-first plan

Add causal gates and prove the old timing-only sequence is eliminated; retain the intended durable ownership assertions.