feat(ai-proxy): add passthrough protocol for unrecognized API formats

[shreemaan-abhishek]

Description

The ai-proxy plugin currently rejects any request whose body does not match a known protocol (openai-chat, openai-responses, openai-embeddings, anthropic-messages, bedrock-converse), making it impossible to proxy OpenAI-compatible APIs whose body shape isn't recognized — for example /v1/images/generations, whose {prompt, model, n, size} body matches no existing matcher. This PR adds a passthrough protocol adapter that is registered last in the detection order and matches any non-empty JSON body, acting as a catch-all. When selected, the plugin forwards the request body unchanged to the upstream using ctx.var.uri (or override.endpoint when configured) as the target path; only model rewrite (options.model), auth header injection (auth.header), and endpoint override (override.endpoint) are applied. All other protocol hooks (SSE parsing, usage extraction, message manipulation, response text extraction) return safe no-op values, so passthrough imposes zero parsing cost and stays out of the way of arbitrary upstream payloads.

Which issue(s) this PR fixes:

Fixes #

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)