docs: add testing resources to llms indexes - #4509

Summary

• add the Testing guide to both LLM documentation indexes
• add Agent, Realtime, and Voice testing API references to the full index
• make provider-neutral testing utilities easier for LLM-based tools to discover

Test plan

• uv run docs/scripts/generate_ref_files.py
• uv run mkdocs build (the two commands in make build-docs; run individually because make.exe is unavailable on this Windows host)
• verified that all newly indexed documentation URLs return HTTP 200
• git diff --check origin/main...HEAD
• codex review --base origin/main — no findings

Issue number

N/A — documentation discoverability update

Checks

• I've added new tests, if relevant — not applicable; this is a docs-only change with no runtime behavior.
• I've run .agents/skills/code-change-verification/scripts/run.sh — not applicable; the repository's docs-only exception applies, so the runtime format/lint/typecheck/test stack was not run.
• I've confirmed all applicable verification steps pass
• If using Codex, I've run /review before submitting this PR