docs(callbacks): correct Python callback signatures and return types

[GWeale]

The callback pages (plus the callback examples in the safety guide) described Java/Kotlin shapes as if they were Python, and one example used a signature the framework never calls.

Changes

• docs/safety/index.md — the before_tool_callback example took a leading callback_context parameter; ADK invokes before-tool hooks with (tool, args, tool_context), so registering that function as written raises TypeError. Parameter dropped and state read off tool_context.state, which also replaces the tool_context.invocation_context.session.state chain in two nearby snippets.
• docs/callbacks/index.md — the return-value section gave Kotlin's CallbackChoice.Continue(...) / Break(...) and Java's Map<String, Any> as the Python contract; it now says None, Content, LlmResponse, and dict.
• docs/callbacks/types-of-callbacks.md — on_model_error_callback and on_tool_error_callback were undocumented; added with their arguments and their exception-suppression behavior. Also notes that every callback field accepts a list (invoked in order, first truthy return wins), that a callback may be def or async def, and that after_tool_callback receives the tool's raw return value — the {"result": ...} wrap happens after the callback runs, so a tool annotated -> str hands the callback a str.
• docs/callbacks/design-patterns-and-best-practices.md — save_artifact / load_artifact were shown without await; "callbacks execute synchronously" reworded to say the loop waits for each one and awaits async def callbacks; the "use the correct context type" advice now notes the two Python names are aliases of the same class.

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.

[netlify]

✅ Deploy Preview for adk-docs-preview ready!

Name	Link
🔨 Latest commit	631e757
🔍 Latest deploy log	https://app.netlify.com/projects/adk-docs-preview/deploys/6a6a9989162dec00088531e9
😎 Deploy Preview	https://deploy-preview-2016--adk-docs-preview.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
🤖 Make changes	Run an agent on this branch

---

To edit notification comments on pull requests, go to your Netlify project configuration.