docs(callbacks): align callback contracts with runtime behavior

[jaywang172]

Link to Issue or Description of Change

Companion user guide: google/adk-docs#2207

The callback API docstrings have drifted from the runtime: before/after agent
and model callbacks stop on truthy responses, and agent callbacks support
positional argument fallback even though the docs require a particular name.

Align the existing docstrings with the current contracts:

• Describe keyword-first binding, positional fallback, and the requirement to
  use canonical names for keyword-only parameters.
• Document truthy stopping for before/after agent and model callbacks.
• Specify LlmResponse for model responses and error recovery.
• Make non-None stopping explicit for tool callbacks, including {} as a
  valid response that skips the tool, replaces its result, or handles an error.

Only docstrings change; callback execution and public signatures are unchanged.
This is independent of #7008.

Testing Plan

• Python 3.11: 214 existing tests passed across the callback pipeline, agent
  fields, base agent, model callbacks, and regular/live tool callbacks.
• Executed the companion guide's four callback signature examples against the
  current helper: canonical names, positional aliases, and canonical
  keyword-only parameters work; a renamed keyword-only parameter raises
  TypeError as documented.
• Compared both files' ASTs after removing docstrings: executable AST unchanged.
• pre-commit run --files src/google/adk/agents/base_agent.py src/google/adk/agents/llm_agent.py: passed.
• git diff --check: passed.

No model requests or Google credentials were needed. No new runtime tests were
added for this documentation-only change.

Checklist

• Read CONTRIBUTING.md and performed a self-review.
• Verified the documentation against the implementation and existing tests.
• Updated the user-facing callback guide in a companion adk-docs PR.