Synchronize SAVEPOINT replay cancellation before mock RPC release

[apstndb]

Fixes #973.

TestSavepointReplayCancellationCleansUpWithoutCommit cancelled the command and immediately closed releaseSQL. That opened the mock's successful-response path before the cancelled context necessarily reached prepareSQL, so reconstruction could finish with err == nil (reconstruction error = <nil> on 0f7522b).

The heartbeat RPC harness already selects on ctx.Done() while the candidate SQL is blocked. This change keeps that barrier closed after cancel() so the in-flight RPC can only leave via cancellation. t.Cleanup still closes releaseSQL if the test fails or times out.

Assertions are unchanged: assigned candidate identity, cancelled cause, ended logical owner, cleared recovery, candidate rollback, and no Commit. No production code change, no sleeps or assertion retries, and draft #947 is untouched.

Based on origin/main f10a4da after an additive merge of #972.

  

[github-actions]

Code Metrics Report

📊 View detailed coverage report (available for 7 days)

	main (ec4e0be)	#975 (8ef92e4)	+/-
Coverage	86.7%	86.7%	+0.0%
Code to Test Ratio	1:2.2	1:2.2	+0.0
Test Execution Time	1m51s	1m54s	+3s

Details

  |                     | main (ec4e0be) | #975 (8ef92e4) |  +/-  |
  |---------------------|----------------|----------------|-------|
+ | Coverage            |          86.7% |          86.7% | +0.0% |
  |   Files             |            112 |            112 |     0 |
  |   Lines             |          10405 |          10428 |   +23 |
+ |   Covered           |           9029 |           9051 |   +22 |
+ | Code to Test Ratio  |          1:2.2 |          1:2.2 |  +0.0 |
  |   Code              |          23263 |          23309 |   +46 |
+ |   Test              |          52022 |          52940 |  +918 |
- | Test Execution Time |          1m51s |          1m54s |   +3s |

Code coverage of files in pull request scope (88.7% → 88.8%)

Files	Coverage	+/-	Status
internal/mycli/directed_read.go	97.0%	+2.9%	affected
internal/mycli/execute_partitioned.go	90.6%	-0.9%	affected
internal/mycli/session.go	85.1%	+0.0%	affected
internal/mycli/transaction_manager.go	87.8%	+0.0%	affected
internal/mycli/var_defs.go	96.0%	+0.0%	affected

---

Reported by octocov

[apstndb]

Independent review of HEAD d3ecdbf04ef72c356c56a0c168e3e00687f005b2: no actionable findings. Keeping the successful-response barrier closed after cancellation removes the competing success path while preserving failure cleanup and every cancellation/ownership assertion.

Independently validated this exact HEAD: the cancellation test passed 50 repetitions, the race-enabled test passed 10 repetitions, and make check passed with Docker. All nine applicable CI checks passed; the conditional Dependabot automerge job is skipped. No unresolved review feedback remains.