fix(log): drain loki hook channel on shutdown

[cursor]

What?

Drain lokiHook's buffered channel on ctx cancel before the final push, so --out loki does not lose a tail of logs that Fire already accepted.

Extracts the per-entry buffering into an appendEntry helper (same logic as before) and reuses it from a non-blocking drainQueued on the shutdown path — matching how the file log hook already drains.

Why?

Listen's ctx.Done() branch pushed only entries already copied into its msgs buffer. Anything still sitting in h.ch was dropped.

Trigger: queue log entries into the hook channel, then cancel the listen context before the run loop drains them (common at test end when many lines are still buffered).

Impact: silent loss of the final batch of --out loki logs.

TestLokiHookDrainsQueuedEntriesOnShutdown fails on unpatched code (all 100 pre-queued messages missing) and passes with this drain.

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

• Same shared-hook drain as discussed in Stream --local-execution logs to Grafana Cloud #6171 (comment) and implemented in Fix loki shutdown log loss and surface dropped cloud logs #6232 (stacked on Stream --local-execution logs to Grafana Cloud #6171).
• This PR lands the master-applicable --out loki portion now; Fix loki shutdown log loss and surface dropped cloud logs #6232 can keep the cloud dropped-log notice that depends on Stream --local-execution logs to Grafana Cloud #6171's Drain() path.

Validation

• go test ./internal/log/ -count=1 passes
• Regression test fails without the drain and passes with it

  

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