feat(llm): compat endpoints declare their JSON mode

[hung12ct]

Problem

Two overlapping defects in the OpenAI adapter's compatible-endpoint path.

1. json_schema was sent unconditionally. GenerateStream set response_format {type:"json_schema"} whenever a StructuredOutput was on the context. Endpoints that publish only the older {type:"json_object"} reject that outright, so every schema-constrained call 400'd while plain chat and tool calling kept working — the failure lands exactly on planner/extractor/judge stages and nowhere else.

2. Capabilities() claimed features on every gateway's behalf. It was a method on the shared *Provider type returning a hardcoded {ImageInput: true, StructuredOutput: true}, and NewCompat returns that same type. CapabilityProvider exists so a consumer can reject an unsuitable provider at construction instead of discovering the gap from a confident, wrong answer — for compatible endpoints it produced exactly that: a pre-flight check that passed, then a failure mid-run.

Change

JSONMode with three values, declared per endpoint:

Mode	Wire behavior
JSONModeSchema (zero value)	response_format {type:"json_schema"}, schema inline, enforced server-side
JSONModeObject	response_format {type:"json_object"} + schema appended as a trailing system message
JSONModeNone	Structured request fails with an error naming the option, before anything is sent

Capabilities() derives from configuration (StructuredOutput: p.jsonMode != JSONModeNone), so one source of truth governs both the claim and the wire — they cannot drift. New is unchanged and still reports both; NewCompat prepends JSONModeNone + no image claim, with caller options winning.

Two details are load-bearing and commented as such:

• The rendered instruction always contains the literal word JSON — endpoints in this mode commonly reject a request whose messages never mention it.
• The schema message is appended last rather than merged into an existing system prompt: it is an instruction, and the final message is the one models follow most closely.

Breaking

Callers using structured output through NewCompat must add WithJSONMode(...). No in-tree callers outside the package. The alternative — defaulting compat to json_schema — preserves both the false capability claim and the 400.

JSONModeObject guarantees only that the reply parses as JSON; Strict degrades from a rule to a request. Documented in docs/providers.md. The adapter deliberately does not validate-and-retry: how many attempts a malformed reply is worth is caller policy.

Tests

Wire-level via captureOpenAIRequest: object mode sends json_object and not json_schema; the trailing system message carries schema + description + the word JSON; no response_format without a schema in any mode; JSONModeNone fails before reaching the httptest server (asserts the server was never called, not merely that an error came back); capability table-test across all three modes plus caller-option precedence.

gofmt -l . empty · make vet · make lint (0 issues) · make build · make test · make test-race — all clean.