fix: pkg/snipe reports the actual landed AZ, not the requested placement

[scttfrdmn]

Summary

• pkg/snipe's Result.AvailabilityZone now reports the AZ the instance actually landed in (spawnaws.LaunchResult.AvailabilityZone, read from the real RunInstances response), instead of the requested placement's AZ from the retry loop's variable.
• Root cause: launchAcrossPlacements (pkg/snipe/snipe.go) returned p.AZ on success — the loop variable holding the REQUESTED placement — rather than result.AvailabilityZone. These coincide whenever a Placement pins a specific AZ (the request forces the match), which is exactly why every existing test passed: they all pin a Placement. The bug only surfaces when Target.Placements is empty and EC2 chooses the AZ itself — a scenario Target.Placements's own doc comment explicitly describes as supported ("Empty = a single AZ-unpinned attempt per round (EC2 chooses the AZ and its default subnet)").
• Verified the fix is correct against spawn's actual RunInstances response mapping: spawnaws.newLaunchResult (spawn's pkg/aws/client.go) sets LaunchResult.AvailabilityZone directly from instance.Placement.AvailabilityZone in the API response — so it is genuinely the landed AZ, not an echo of the launch config. No live AWS call was needed to confirm this; the mapping is unambiguous in spawn's source.

Closes #114.

Changes

• pkg/snipe/snipe.go: launchAcrossPlacements returns result.AvailabilityZone instead of p.AZ.
• pkg/snipe/snipe_internal_test.go:
  • Added TestSnipe_ReportsActualLandedAZWhenEC2Chooses — the regression guard, with no Placements set and a fake provide that returns a landed AZ the request never named.
  • Updated two existing fakes (TestSnipe_AcquiresAfterCapacityRetries, TestSnipe_SubnetPerPlacement) to populate LaunchResult.AvailabilityZone, mirroring a real RunInstances response — previously harmless since the placement AZ was returned regardless, now load-bearing.
• CHANGELOG.md: [Unreleased] entry under Fixed.

Note

The same bug pattern exists in the legacy pkg/watcher.Spawner.launchAcrossAZs (pkg/watcher/spawner.go:241, returns the requested AZ a rather than result.AvailabilityZone). Left untouched here — issue #114 scopes to pkg/snipe, and per the [0.52.0] CHANGELOG entry watcher.Snipe/Spawner.Snipe have no current callers in this repo (kept as-is rather than rewired). Filing this note here for visibility; happy to open a follow-up issue if wanted.

Verification

• Confirmed the new test fails without the fix (git stash the source-only change) and passes with it restored.
• go build ./..., go vet ./..., go test ./... clean from repo root.
• go build ./..., go vet ./..., go test ./... clean from lambda/capacity-poller (untouched, confirmed still green).
• golangci-lint run ./pkg/snipe/... — 0 issues.

Test plan

• CI green

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!