fix(responses): default text.format for openai-compatible responses providers (#2093)

[StevanusPangau]

Problem

When using a custom OpenAI-compatible provider configured with API Type responses, 9Router forwards a request body whose text is an object missing text.format. Some Responses-compatible upstreams (e.g. LM Studio) reject this with:

{ "error": { "message": "Required", "type": "invalid_request_error", "param": "text.format", "code": "missing_required_parameter" } }

This happens because a client calling /v1/responses (source format openai-responses) routed to an openai-compatible responses provider (target format openai-responses) hits the translator passthrough path — source === target, so translation is skipped and the client's text object is forwarded verbatim, without a default text.format.

Fix

Default text.format to { type: "text" } (the Responses API default for that field) in the executor before forwarding upstream, scoped to openai-compatible responses providers:

• Only applies when the provider id is openai-compatible-* and includes responses.
• Only applies when body.text is a plain object without a format.
• Builds a new text object instead of mutating the caller's, and never overwrites an existing format.

Why this is safe

• Scoped to openai-compatible responses providers; official OpenAI/codex/github executors are untouched.
• No-op when text is absent, non-object, or already has format.
• { type: "text" } is the documented Responses API default, so it does not change behavior for upstreams that were already accepting the request.

Tests

Adds tests/unit/openai-compat-responses-text-format.test.js:

• Defaults text.format to { type: "text" } when text is an object without format.
• Preserves an existing text.format.
• No-op when there is no text.
• Does not touch a non-responses (chat) openai-compatible provider.
• Ignores a non-object text value.

npm exec -- vitest run tests/unit/openai-compat-responses-text-format.test.js --config tests/vitest.config.js
npm run build

All pass; related executor unit suites stay green (no regressions).

Fixes #2093