docs: document AsyncPipeline task cancellation and sync streaming callbacks

[julian-risch]

Related Issues

• Resolves two subtasks of deepset-ai/haystack-private#381 (documentation updates needed for Haystack 3.0)

Proposed Changes:

Documents two of the smallest subtasks of the tracking issue, ranked by effort:

• fix: In AsyncPipeline cancel inflight tasks if a PipelineRuntimeError is raised by another component #11499 — AsyncPipeline docs: new "Error Handling and Task Cancellation" section explaining that in-flight tasks are cancelled and drained when a component raises, when run_async_generator iteration is abandoned, or when the run is cancelled — plus the caveat that sync components offloaded to a worker thread run to completion with their outputs discarded.
• refactor: accept sync callbacks in run_async #11466 — Streaming docs: new "Sync and Async Callbacks" section in the Choosing the Right Generator guide (with a runnable example) explaining that sync streaming_callbacks are now accepted in async contexts with a logged warning, while async callbacks in sync run still raise. Cross-linked from the AsyncPipeline page.

Note: this PR originally also carried the SearchableToolset duplicate-name note (#11549). That change was moved to #11873, which contains all other SearchableToolset docs updates, so the two PRs no longer conflict on the same file.

How did you test it?

npm run build in docs-website/ passes (also catches broken links). The sync-callback behavior (streaming proceeds, warning logged) was executed against the v3 code with an offline generator.

Notes for the reviewer

Draft PR targeting v3. Content is based on the release notes of the two code PRs.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have run pre-commit hooks and fixed any issue.

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Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ready	Preview, Comment	Jul 5, 2026 8:39pm