docs: explain function tool pre-execution guardrails

[adityasingh2400]

Summary

• Document tool_input_guardrails as the current pre-execution validation hook for local function tools.
• Include a concrete example for rejecting or halting a tool call based on the final serialized arguments.
• Clarify that Python-side guardrails cannot intercept hosted tools because those execute server-side.

Test plan

• uv run mkdocs build
• git diff --check

Note: uv run mkdocs build --strict currently aborts on pre-existing translated-doc/autoref warnings unrelated to this change.

Closes #2970.

[seratch]

This is already covered by the guardrails document page.