feat(ai-proxy): add per-protocol request_body override and rename max_tokens mapping to llm_options

[nic-6443]

Description

This PR brings back the per-protocol override.request_body deep-merge capability (from the predecessor of #13251) while keeping the provider-aware max_tokens mapping — renamed to override.llm_options.

Why

Users want both override mechanisms:

• llm_options: Simple, provider-aware max_tokens mapping (set one value, APISIX maps it to the right field per provider). Always force-overwrites.
• request_body: Fine-grained, per-protocol deep-merge for arbitrary fields like temperature, stop_sequences, stream_options, etc.

Changes

• Rename override.request_body → override.llm_options (always force-overwrites, no force flag)
• Restore override.request_body as per-protocol deep-merge keyed by target protocol (openai-chat, openai-responses, etc.)
• Add override.request_body_force_override (boolean, controls request_body deep-merge only)
• Precedence: model_options → llm_options → request_body
• Recreate merge.lua with force-aware deep_merge
• Restore protocols.names() for dynamic schema enum

Config example

{
  "override": {
    "llm_options": { "max_tokens": 500 },
    "request_body": {
      "openai-chat": { "temperature": 0.2, "stop": ["Human:"] },
      "anthropic-messages": { "max_tokens": 500 }
    },
    "request_body_force_override": false
  }
}

[Copilot]

Pull request overview

Reintroduces per-target-protocol override.request_body deep-merge for ai-proxy/ai-proxy-multi while renaming the provider-aware max_tokens override mechanism to override.llm_options and documenting the new precedence/semantics.

Changes:

• Add override.llm_options (provider-aware mapping; always force-overwrites) and restore override.request_body as a per-target-protocol deep-merge map with request_body_force_override.
• Introduce a shared deep_merge helper and apply overrides in the order: model_options → llm_options → request_body.
• Update schemas, docs (EN/ZH), and add/adjust tests to cover the new configuration and behaviors.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
t/plugin/ai-proxy-request-body-override.t	Expands test coverage for llm_options mapping + per-protocol request_body deep-merge semantics and precedence.
docs/zh/latest/plugins/ai-proxy.md	Updates ZH docs for llm_options + per-protocol request_body override semantics and precedence.
docs/zh/latest/plugins/ai-proxy-multi.md	Updates ZH multi-instance docs to reflect llm_options + per-protocol request_body.
docs/en/latest/plugins/ai-proxy.md	Updates EN docs for renamed llm_options and restored per-protocol request_body override behavior.
docs/en/latest/plugins/ai-proxy-multi.md	Updates EN multi-instance docs to reflect llm_options + per-protocol request_body.
apisix/plugins/ai-proxy/schema.lua	Extends schema with override.llm_options and protocol-keyed override.request_body validation.
apisix/plugins/ai-proxy/merge.lua	Adds deep_merge helper used to apply per-protocol request body patches.
apisix/plugins/ai-proxy/base.lua	Passes new override fields (llm_options, request_body map, force flag) into provider request building.
apisix/plugins/ai-providers/base.lua	Applies llm_options via provider hook (forced) and applies per-protocol request_body via deep merge.
apisix/plugins/ai-protocols/init.lua	Restores protocols.names() to enumerate registered protocols for schema building.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.