docs(examples): fix unrunnable Python snippets included by the docs

[GWeale]

The examples/python/** snippets are pulled into the docs with --8<--, so a
broken snippet renders as broken documentation. 14 of them did not run, and one
did not even parse.

Verified against google/adk-python @ main by importing and introspecting every
symbol, not by reading. Compile sweep over examples/python/**: 50/64 files
parsed before, 64/64 after.

What was wrong

• tools/overview/doc_analysis.py — await inside a plain def, so the whole
  file was a SyntaxError; load_artifact and search_memory are coroutines
  and were not awaited; MemoryEntry.events does not exist (the field is
  content).
• tools/overview/customer_support_agent.py — sub_agents assigned after
  construction, which skips model_post_init's parent linking, so
  parent_agent stayed None.
• tools/auth/agent_cli.py, tools/built-in-tools/agent_search.py —
  create_session is a coroutine and was not awaited, so the run raised
  SessionNotFoundError.
• tools/auth/tools_and_agent.py — tools=toolset.get_tools() passed a
  coroutine, raising TypeError: 'coroutine' object is not iterable.
• tools/auth/helpers.py — read the function-call arg auth_config, but ADK
  serialises with by_alias=True, so the key is authConfig; the helper always
  raised.
• tools/built-in-tools/{spanner,bigtable}.py — sync wrappers returned an
  un-awaited coroutine from execute_sql.
• tools/built-in-tools/agent_search.py — grounding_attributions does not
  exist on types.GroundingMetadata.
• callbacks/{before,after}_agent_callback.py — event.is_error() and
  event.error_details have never been part of the Event API.
• callbacks/after_agent_callback.py — comments said the callback replaces
  the agent's output; it appends a new event.
• callbacks/after_tool_callback.py — comments said tool_response is
  {"result": <value>}; the callback receives the raw return value.

Also in this PR

• Files presented as complete runnable code no longer have module-level
  await; the driver is wrapped in async def main() under
  if __name__ == "__main__", matching the pattern already used in each
  directory.
• 13 unguarded event.content.parts[0].text dereferences under
  is_final_response() now carry the guard already used elsewhere in this repo.
  is_final_response() never inspects content, so the unguarded form crashes
  on safety-blocked turns, state-only events and grounding-only responses.
• tutorial/agent_team/adk_tutorial.ipynb — the same four defects corrected in
  docs/tutorials/agent-team.md, so the notebook and the page agree.

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