docs(plugin): explain LLM request options and SDK 0.7.12 cancellation

[RockChinQ]

Document the LLM request-layer changes in Chinese, English and Japanese plugin development guides. Cover request_policy precedence and deadlines, retry limits, numeric reasoning budgets, prompt caching, attribution/session headers, complete usage, monitoring diagnostics and deterministic stream closure.

Link the new guide from the common API and context references and add localized navigation/sitemap entries. Explicitly identify SDK 0.7.12 as the released early-close fix and explain that separately deployed Runtime/plugin environments also need upgrading. Preserve existing plugin/Runner signatures and legacy error transport.

Validation:

• New examples run against published SDK 0.7.12 with a mock Host; all three locales share identical Python, retry parameters are preserved, and early close reaches the Host generator.
• Node documentation tests: 38 passed. Python documentation tests: 18 passed, 1 skipped (run explicitly with python because Windows has no python3 command).
• Static-site validation: 31 passed after updating expected counts for the three new pages. Final npm run build (including all postbuild steps and static checks) succeeded.
• Full legacy example verification exposes an existing list_parsers call bound to RunnerAPIProxy, which does not expose that API. The new guide is verified separately with --page llm-requests; this PR does not claim the old examples all pass.

Related: langbot-app/LangBot#2661, langbot-app/LangBot#2663, langbot-app/langbot-plugin-sdk#177, langbot-app/langbot-plugin-sdk#178.