fix: preserve user downloadsPath and reject non-positive http.batch

[cursor]

What?

Two high-severity correctness fixes:

1. Browser downloadsPath data loss: BrowserContext.cleanup() always called os.RemoveAll on DownloadsPath, including user-supplied paths. On browser.close(), a path like /tmp or a project downloads directory was recursively deleted. Only temporary directories created by k6 are removed now (same ownership pattern as storage.Dir).

2. http.batch hang: With options.batch <= 0 (or K6_BATCH=0), MakeBatchRequests spawned zero workers and http.batch() blocked forever waiting for results. Non-positive batch / batchPerHost are rejected in Options.Validate(), and MakeBatchRequests fails closed instead of hanging.

Why?

• Data loss: user-facing downloadsPath is a supported BrowserContext option; unit tests even use /tmp as an example value. Closing the browser must not wipe that tree.
• Hang: batch: 0 is accepted by config today and turns any http.batch([...]) into a permanent VU deadlock until scenario timeout.

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.

Targeted validation run:

• go test -race ./internal/js/modules/k6/browser/common/ -run TestSetDownloadsPath
• go test -race ./lib/ -run TestOptions
• go test -race ./lib/netext/httpext/ -run TestMakeBatchRequests

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)

Found by critical-bug investigation automation on tip 53b5727d8.

  

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