test: fix recurring CI flakes in desktop and Postgres lanes (#8343)

Fixes the CI flakes that failed most often across `main` and unrelated
branches from Oct 5 to Oct 14. The counts come from 150 failed `CI`
runs. Every change except the nextest config is test-only.

| Flake | Runs hit | Root cause | Fix |
|---|---|---|---|
| `probe_codex_acp_version_…descendant_holds_pipe_open` | 5 | The test
writes its script in-process and then execs it. If a sibling test spawns
a process in that window, the child inherits the open write fd and Linux
rejects the exec with `ETXTBSY`. The probe swallows that error as
`None`. | Write probe scripts from a short-lived `/bin/sh`
(`write_executable_script`) |
| `empty-edit-delete.spec.ts` (`:51`, `:74`, `:95`) | 11 | The seeded
`mock-general-welcome` id isn't hex, so `formatTimelineMessages` can't
overlay its edit. The post-subscribe window refetch then reverts the
row. | Emit a per-test message with a real id after the exact channel
subscription is live |
| `huddle-transcription.spec.ts:798` | 9 | The composer's delayed
initial autofocus (TipTap `focus()` runs in rAF) lands after the voice
popover opens and dismisses it. | Wait for `message-input` focus before
opening the popover |
| `voice-note.spec.ts:906` | 6 | The fixture `ping.mp3` is about 1s
long, so playback can end before `paused === false` is observed. | Loop
the fixture audio |
| `workflows.spec.ts:291` | 5 | The `Message text` fill runs before the
inspector swaps to the new step, so it fills the trigger's condition
instead. | Scope the fill to the step inspector after its `Action` menu
appears |
| `cluster_global_probe_commits_tokens_and_sessions_prove_coverage` | 3
| The `max-threads = 1` group only serializes these tests with each
other. Other tests' sessions still mask cluster-wide `pg_stat_activity`
(`MaskedActivity`). | `threads-required = "num-test-threads"` |
| `test_usage_metrics_lock_has_single_owner_and_releases_on_drop` | 1
(this PR) | The test retries the lock right after dropping the leader.
Postgres releases a session advisory lock only after the backend notices
the disconnect, so the retry can run first. | Poll for the successor,
bounded at 5s |

### Validation
- **ETXTBSY:** a Linux experiment with Rust 1.95 using the same
write→chmod→spawn pattern under concurrent spawns hit `ETXTBSY` 4–7% of
the time. Using the `sh` writer, it hit 0 of 6,000. I did not catch it
inside the full suite locally, so this fix is based on the mechanism,
not an in-suite repro. All 11 probe tests pass on Linux and macOS.
- **empty-edit-delete:** the baseline failed 16 of 45 runs
(`--repeat-each`, 6 workers, no retries). The spec fix alone brings that
to 4/45. The remaining failures come from a separate product race,
described below.
- **huddle:** the baseline failed 5 of 10 runs. With the fix, 60 of 60
passed without retries.
- **nextest:** checked with a throwaway crate on 0.9.136. A grouped test
with `num-test-threads` ran alone, even with `max-threads = 1`.
- I have not stress-run the voice-note and workflows fixes; they rely on
CI.

### Not in this PR
- **Product bug in `useChannelLinks`:** the debounced `#channel`
autocomplete close lets Enter accept a stale suggestion for about 120ms.
That accounts for the remaining `empty-edit-delete` failures, and it
affects users too. With the spec fix plus a synchronous close, the spec
passed 120 of 120 runs. This needs a product decision, so it's held for
a separate PR.
- **Composer autofocus race:** the delayed autofocus can close an
overlay the user just opened. The real fix is to revalidate when the
delayed focus runs. Overlaps #6530.
- **`thread-head-stale-edit.spec.ts:55`:** a forced click on a
pointer-events-none action bar is suspected, but I couldn't reproduce
it.
- **Remaining ETXTBSY exposure:** about 40 other Rust tests write and
then exec scripts, and 17 specs still edit the non-hex seed message.

Root-cause lanes by Princess Donut, Mongo and Mordecai. Carl integrated
and verified them.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <32a2e2c9d428ee08902cab75d956da2c1d235a22d4766b0dd4138bf6e2e5db1d@buzz.block.builderlab.xyz>
Co-authored-by: Princess Donut <5d97ac8c272fa949af56c71e586146b1706e5c4c3daa125db5b4625e12e0686c@buzz.block.builderlab.xyz>
Co-authored-by: Mongo <81cabd2ca1792494372ed410283c00770c0cf35afca052e940fa522883212b45@buzz.block.builderlab.xyz>
Co-authored-by: Mordecai <f314b6033b6e13df94c8e9e1187d3666b6b9ce54e66ee986e31cdaa909e8dcbc@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/buzz-db/src/store/usage.rs

@@ -928,14 +928,24 @@ mod postgres_tests {
         );
 
         drop(leader);
-        assert!(
-            second
+        // Dropping closes the socket; the server releases the session lock only
+        // once its backend observes the disconnect, so poll briefly.
+        let deadline = tokio::time::Instant::now() + std::time::Duration::from_secs(5);
+        let successor = loop {
+            let attempt = second
                 .try_lock_usage_metrics(key)
                 .await
-                .expect("lock attempt after leader drop")
-                .is_some(),
+                .expect("lock attempt after leader drop");
+            if attempt.is_some() || tokio::time::Instant::now() >= deadline {
+                break attempt;
+            }
+            tokio::time::sleep(std::time::Duration::from_millis(20)).await;
+        };
+        assert!(
+            successor.is_some(),
             "dropping the detached session releases its advisory lock"
         );
+        drop(successor);
 
         // Release any remaining session state before DROP DATABASE.
         drop(first);