docs(guardrails): add pre-execution policy example

[EfeDurmaz16]

Summary

This adds a concrete tool input guardrail example for pre-execution policy checks, following the current recommended extension point for validating a local function tool call immediately before invocation.

The example shows a payment policy guardrail that checks the exact requested tool payload before charge_card executes:

• tool name allowlist
• merchant allowlist
• amount limit
• model-visible rejection via reject_content

It also calls out the important boundary that Python-side tool input guardrails apply to local function tools, not hosted tools that execute server-side.

Closes #2970.

Verification

• uv run python -m py_compile examples/basic/tool_policy_guardrail.py
• uv run ruff check examples/basic/tool_policy_guardrail.py
• uv run pyright examples/basic/tool_policy_guardrail.py
• uv run pytest tests/test_tool_guardrails.py
• git diff --check

Note: uv run mkdocs build --strict currently fails on existing repository-wide i18n/autoref warnings unrelated to this change.

[seratch]

Thanks for the suggestion, but we already cover the tool guardrail patterns in the docs.