Add OpenAI-compatible llm.Provider adapter

internal/ai/llm/openai implements Provider over plain net/http against
POST {base}/chat/completions and GET {base}/models, with Bearer auth and
optional extra headers. Structured output uses response_format
json_schema (strict), json_object, or prompt-only with first-JSON-object
extraction, per the configured mode. Usage tokens, model and
finish_reason are parsed.

Errors map 401/403 to ErrAuth, 429 to ErrRateLimited with Retry-After
(seconds or HTTP-date), 408 and transport timeouts to ErrTimeout, and
other non-2xx statuses (and OpenRouter error bodies in a 2xx) to
ErrProvider with a key-redacted excerpt.

Contract tests replay OpenAI, OpenRouter and Ollama /v1 payloads from
httptest servers and assert the key and Authorization value never
appear in an error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/openai/testdata/ollama_models.json

@@ -0,0 +1,7 @@
+{
+  "object": "list",
+  "data": [
+    {"id": "llama3.2:latest", "object": "model", "created": 1741000000, "owned_by": "library"},
+    {"id": "qwen2.5:7b", "object": "model", "created": 1740000000, "owned_by": "library"}
+  ]
+}