docs(streaming): fix inverted env var, wrong defaults, broken snippets

[GWeale]

Corrections to the docs/streaming/ pages: snippets that can't run, an environment variable documented with inverted polarity, and several defaults documented backwards.

Changes

• docs/streaming/dev-guide/part4.md — progressive SSE streaming was documented as off-by-default and enabled with export ADK_ENABLE_PROGRESSIVE_SSE_STREAMING=1; it is on by default and turned off with ADK_DISABLE_PROGRESSIVE_SSE_STREAMING=1. Also: response_modalities is list[Modality], not list[str]; run_live() does not read streaming_mode (that selects NONE vs SSE for run_async()); noted the reconnect loop re-raises after 5 consecutive failures; a run_live(session=...) call now passes user_id / session_id.
• docs/streaming/dev-guide/part3.md — the tool example was a non-async def containing await, and took an InvocationContext; it is now async def taking a ToolContext, using tool_context.state and tool_context.save_artifact(), with a warning that a tool cannot set end_invocation (it only gets a copy of the InvocationContext). Also: a streaming-display snippet was gated on if not event.partial: (that's the final merged event, not the incremental ones); transcriptions were described as always complete when incremental chunks arrive with partial=True; audio artifact saving and cache flushing are gated on RunConfig.save_live_blob; transcription is on by default; aggregation happens in GeminiLlmConnection.receive(), not a StreamingResponseAggregator; streaming-tool queues are registered lazily on first call rather than up front, and the line-number code references were replaced with function names; fixed an indentation error that placed await play_audio(...) outside its loop.
• docs/streaming/dev-guide/part2.md — LiveRequest was missing its partial and state_delta fields; the claim that ADK auto-creates an event loop for LiveRequestQueue is wrong (nothing is created — the underlying asyncio.Queue binds late, on first wait); and the automatic close() applies to ADK's internal CFC queue, not to a queue you create and pass to run_live().
• docs/streaming/dev-guide/part1.md — the Gemini API version was listed as v1beta; it is v1alpha. A missing session raises SessionNotFoundError (a ValueError subclass), and Runner(..., auto_create_session=True) avoids it.
• docs/streaming/configuration.md — added that input_audio_transcription / output_audio_transcription default to AudioTranscriptionConfig(), not None, so both sides are transcribed unless you opt out, plus a pointer to the full RunConfig reference.
• docs/streaming/dev-guide/part5.md — two Agent(...) snippets omitted the required name=.
• docs/streaming/streaming-tools.md — Part.from_text(prompt_text) → Part.from_text(text=prompt_text); the argument is keyword-only.

How this was produced

Part of a page-by-page audit of the Python docs against the google/adk-python v2.5.0 source: every import resolved against a real 2.5.0 install, every constructor kwarg checked against model_fields / inspect.signature, every documented default read off the field. A second independent pass re-derived each claim from source rather than trusting the finding, and a third conformed the new wording to the surrounding pages. mkdocs build --strict is clean.

Only Python tabs and language-neutral prose were touched — this audit had no ground truth for the Go / Java / TypeScript SDKs.

Split out of a larger audit branch so each area can be reviewed on its own.

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