docs: document Pipeline.stream(), ChatPromptBuilder insert tag, and Toolset run-scoped copies

[julian-risch]

Related Issues

• Resolves three subtasks of deepset-ai/haystack-private#381:
  • feat: expose AsyncPipeline.stream for chunk-by-chunk consumption #11258
  • feat: Extend ChatPromptBuilder with insert tag #11486
  • fix: Fix usage of SearchableToolset with Agent when selecting a subset of tools to be active #11564

Proposed Changes:

• feat: expose AsyncPipeline.stream for chunk-by-chunk consumption #11258 — AsyncPipeline docs: new "Streaming (stream)" execution method section with a runnable example — iterate the returned handle with async for to consume StreamingChunks, read the final output from handle.result, restrict sources via streaming_components, and control abandonment behavior with cancel_on_abandon. Note: the example uses Pipeline, matching the v3 merge of AsyncPipeline into Pipeline; the rest of the (stale) page is left for the MIGRATION.md-driven docs pass, which issue Introduce Preprocessor class that does splitting and cleaning  #381 explicitly does not track.
• feat: Extend ChatPromptBuilder with insert tag #11486 — ChatPromptBuilder docs: new "The insert Tag" subsection under String Templates, documenting {% insert %} with a runnable example, expression/slice/index support, lossless round-tripping of all content types, and multiple-tag usage.
• fix: Fix usage of SearchableToolset with Agent when selecting a subset of tools to be active #11564 — Toolset docs: new "Run-Scoped Copies and Tool Selection" section covering spawn() and get_selectable_tools(), per-run isolation (concurrent runs sharing a Toolset are safe), and runtime name-based selection via Agent.run(tools=[...]) keeping toolsets live. Also updated the SearchableToolset "Reusing the toolset across multiple agent runs" section, which still advised calling clear() between runs — discovered tools no longer persist across Agent runs.

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Project	Deployment	Actions	Updated (UTC)
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