docs(tutorials): fix ToolContext, output_key and persistence claims

[GWeale]

Corrections to the tutorial, quickstart and grounding pages — the ones a new
user runs first. Verified against google/adk-python @ main; all 34 Python
blocks compile and all 3 JSON blocks parse.

docs/tutorials/agent-team.md

• Said ToolContext must be declared last in a tool signature. It does not:
  ADK matches the context parameter on its annotation type at any position
  (utils/context_utils.py:67-79, tools/function_tool.py:116). Proved with
  the parameter first, middle, and renamed.
• A code comment claimed the delegated greeting turn overwrites
  last_weather_report via output_key. It does not —
  agents/llm_agent.py:953-963 early-returns when event.author != self.name.
  The page's own prose 60 lines later already said so.
• Said persistent session storage "requires custom implementation or future ADK
  integrations". DatabaseSessionService and VertexAiSessionService already
  ship.
• A helper annotated -> InMemorySessionService that returns a Session; a
  constant whose name and value disagreed; pip install litellm unpinned when
  ADK requires litellm>=1.84.

docs/get-started/streaming/quickstart-streaming.md

Said Callback, LongRunningTool, ExampleTool and Shell agents "will be supported
in future versions of ADK Streaming". All four work on the live path today —
each verified individually. Only model callbacks are genuinely absent, since
_handle_*_model_callback is reached only from the non-live _call_llm_async.
The caveat now states that limitation instead.

docs/grounding/grounding_with_search.md

The documented groundingChunks[].document{title,uri,id} shape does not exist —
types.GroundingMetadata.model_validate rejects it (extra_forbidden). For
this page's datastore case the real field is retrievedContext. The corrected
payload round-trips; the old one fails validation. Also guards
len(grounding_chunks), which raises TypeError when the field is None.

Smaller

• docs/get-started/python.md — the project layout omitted the .gitignore
  that adk create writes, and the prose did not mention adk create is
  interactive (it prompts for model and backend).
• docs/evaluate/custom_metrics.md — prose named
  check_final_response_match and response_match where the example defines
  check_final_response_exact_match.

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