docs(plugins): fix broken examples, correct plugin callback docs

[GWeale]

The plugins and skills pages: examples that can't run as written, plus callback behavior and Python registration guidance that had drifted from the source.

Changes

• docs/plugins/index.md — the headline CountInvocationPlugin example had its whole class body unindented (docstring, __init__ and both callbacks sat at module level), and the registration example defined root_agent inside the hello_world tool function, so it was never importable; both are now indented correctly. Beyond that: notes that Python's Runner/InMemoryRunner plugins= is deprecated in favor of App(plugins=...) and that passing both raises ValueError; documents on_agent_error_callback, on_run_error_callback and close() (bounded by plugin_close_timeout, five seconds by default); corrects after_run_callback's return annotation from Optional[None] to None; describes the on-event return as a merge onto the original event rather than a wholesale replace, with id, invocation_id and timestamp always taken from the original; adds the built-in Debug Logging plugin to the list; and repairs two garbled "is only supported by the Plugins feature works as follows" sentences.
• docs/skills/index.md — the inline-skills example passed get_weather_tool to SkillToolset without ever defining it, so it could not run; the function is now shown. The skill directory is renamed weather_skill → weather-skill to match the frontmatter name, and the page now states that a filesystem skill's directory name must match that name or loading fails.

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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