bug: ai-prompt plugins have protocol, template-context, and input-validation gaps #14155

Current Behavior\n\nReviewing the existing ai-prompt plugins exposed three problems that predate pull request #14139:\n\n1. ai-prompt-template always emits the configured Chat Completions-shaped object, regardless of the request protocol.\n\n- On /v1/messages, a configured system message remains inside messages, which Anthropic rejects.\n- On /model/.../converse, message content remains a string instead of Bedrock Converse content blocks.\n- On /v1/responses, the generated body has messages rather than input and instructions.\n\n2. Template evaluation inherits unsafe and surprising name-resolution behavior from the current body-transformer path.\n\n- Request fields can shadow Lua globals used by the compiled template. For example, a request field named table can break a template with an error about table.concat.\n- A false request value can fall through during lookup and render as an empty value.\n- An absent variable can resolve to a Lua global instead of being treated as missing.\n\n3. Malformed request shapes can raise Lua exceptions and return 500 responses.\n\n- A top-level JSON number or boolean reaches template_name indexing in ai-prompt-template.\n- A non-string template_name reaches string concatenation.\n- An Anthropic request with messages set to a string can reach ipairs in the protocol adapter.\n\nThese behaviors exist on the base branch without the changes from #14139. They are being tracked separately so fixes for newly introduced regressions in that pull request do not silently expand into an unrelated compatibility redesign.\n\n## Expected Behavior\n\n- ai-prompt-template should either serialize its canonical template into each explicitly supported protocol or reject unsupported protocols with a deterministic 4xx response. Its documented protocol scope should be explicit.\n- Template variable lookup should preserve false values, avoid collisions with runtime globals/helpers, and produce a clear client error for missing variables.\n- The prompt plugins and protocol matchers should validate top-level and nested request shapes before indexing or iterating them. Malformed client input should return a sanitized 4xx response rather than a Lua exception or 500.\n\n## Steps to Reproduce\n\nProtocol output:\n\n1. Configure ai-prompt-template with a template containing model and messages.\n2. Send template parameters through routes using /v1/messages, /model/test/converse, and /v1/responses.\n3. Observe that every generated body keeps the Chat Completions messages shape.\n\nTemplate context lookup:\n\n1. Configure a message containing Describe {{ table }}.\n2. Send a request containing a string field named table.\n3. Observe a 503 caused by the request value shadowing the Lua table global.\n4. Repeat with a false variable or a missing name and observe that the rendered value does not preserve the request semantics.\n\nMalformed input:\n\n1. Send a JSON number or boolean to an ai-prompt-template route.\n2. Send an object whose template_name is an object.\n3. Send an Anthropic-shaped request whose messages value is a string through ai-prompt-decorator.\n4. Observe Lua runtime errors and 500 responses instead of schema-style 4xx errors.\n\n## Acceptance Criteria\n\n- Define and document the protocol scope of ai-prompt-template.\n- Add protocol-specific output or explicit rejection tests for Anthropic Messages, Bedrock Converse, and OpenAI Responses.\n- Define safe variable-resolution semantics, including false and missing values and reserved names.\n- Validate top-level JSON objects, template_name, and protocol message containers before use.\n- Return sanitized 4xx errors for malformed client input.\n- Preserve compatibility intentionally or document and plan any required migration.\n\n## Environment\n\n- APISIX: current master before #14139\n- OpenResty: 1.25.3.2 in the local reproduction environment\n- Reproduced through source inspection and focused Lua tests; the malformed-shape paths are directly reachable in the current modules.