test(infra): migrate integration tests to explicit CleanupGuard cleanup

[Detair]

Summary

Eliminates the recurring CI flake where Rust integration tests intermittently time out at nextest's 120s slow-timeout. Three different tests (later 4) hit this on PRs #512, #513, and #514 — always reported as test #900/900 because nextest reports completion in finish-time order.

Root cause: CleanupGuard::drop (server/tests/integration/helpers/mod.rs) spawned a thread + new tokio runtime + .join().expect() with no timeout. When cleanup hangs (DB pool contention, slow DELETE, lock wait), the test process holds for 120s and gets killed.

Issue #509 documented the same anti-pattern in the setup tests with the same root cause analysis.

Layers

Layer 1 — Stopgap (1 commit, helpers/mod.rs): Bounded join in CleanupGuard::drop. 30-second polling join, then detach. Immediately removes the failure mode.

Layer 2 — Proper fix (1 commit + 16 migration commits, ~136 sites across 16 files): Added pub async fn cleanup(mut self) that consumes the guard and runs cleanup actions on the caller's existing tokio runtime. Migrated all 136 test sites to call guard.cleanup().await; before returning. The Drop fallback stays as a permanent safety net for tests that panic before reaching cleanup (RAII semantics) and prints a "dropped with N pending actions" warning so unmigrated tests are visible.

Layer 3 — Deferred: After ~2 weeks of clean main-branch CI with zero "dropped with N pending actions" warnings, a follow-up commit will replace the Drop fallback's eprintln with panic!() to permanently enforce the contract. Tracked in: #516.

Migration coverage

File	Sites
workspaces.rs	24
filters_http.rs	17
governance.rs	12
guild_limits.rs	10
webhooks.rs	10
channel_pins.rs	10
connectivity_http.rs	8
custom_status.rs	6
setup_http.rs	6
messages_http.rs	6
media_processing.rs	6
bot_intents.rs	6
dm_http.rs	5
channels_http.rs	5
uploads_http.rs	3
setup_concurrent_http.rs	2
Total	136

Test plan

• cargo build --tests -p vc-server clean
• cargo clippy -p vc-server --lib -- -D warnings clean
• cargo nextest run -p vc-server --lib → 430/430 passing
• cargo fmt --check clean
• Per-file grep -c "let mut guard" equals grep -c "guard.cleanup().await" across all 16 files (136/136)
• CI Rust Tests passes 3 consecutive runs on this PR
• No "dropped with N pending actions" warnings appear in CI logs

Refs

• Spec: docs/superpowers/specs/2026-04-11-security-audit-followups-design.md (Topic 4)
• Plan: docs/superpowers/plans/2026-04-11-cleanup-guard-test-flake.md
• Related: Flaky: HTTP-layer concurrent setup tests fail under CI load (root cause TBD) #509 (same anti-pattern in setup tests, root-caused identically)

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