Translate json_schema and drop non-OpenAI fields in OpenRouter adapter

The adapter previously did a blind payload.update(config.model_params).
For callers like activity-core that pass reasoning_effort, max_depth,
and json_schema (Claude / llm-connect-specific fields), those leaked
into the OpenAI Chat Completions request body and OpenRouter rejected
the whole call with HTTP 400. CUST-WP-0045 canary on 2026-06-02 hit
this — manual repro confirmed: same prompt with no model_params returns
a clean 10-recommendation WSJF report in 4.5s; with model_params
included, every call 400s.

Replace the merge with a whitelist + translation step:

- pass-through known OpenAI Chat Completions fields (top_p, stop, seed,
  tools, response_format, etc.)
- translate json_schema into the proper response_format wrapper
  ({type:"json_schema", json_schema:{name,schema,strict}})
- drop documented non-OpenAI fields (reasoning_effort, max_depth) so
  the payload stays valid
- silently drop unknown keys rather than risk another 400

The same pattern will need to apply to the OpenAI and Gemini adapters
when their callers start passing provider-specific keys — left as
follow-up rather than speculative refactoring.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

llm_connect/openrouter.py

@@ -69,9 +69,8 @@ class OpenRouterAdapter(LLMAdapter):
             "temperature": config.temperature,
             "max_tokens": config.max_tokens,
         }
-        # Merge extra model_params from RunConfig
         if config.model_params:
-            payload.update(config.model_params)
+            _merge_model_params(payload, config.model_params)
 
         headers = {
             "Authorization": f"Bearer {self._api_key}",
@@ -140,3 +139,60 @@ class OpenRouterAdapter(LLMAdapter):
                 else:
                     raise
         raise last_exc  # type: ignore[misc]
+
+
+# OpenAI Chat Completions fields that map straight through from model_params.
+# Anything not in this set is provider-specific and must be either translated
+# or dropped — we never blind-merge into the payload, because OpenRouter
+# rejects unknown top-level fields with HTTP 400.
+_OPENAI_PASSTHROUGH_FIELDS = frozenset({
+    "top_p", "n", "stream", "stop", "presence_penalty",
+    "frequency_penalty", "logit_bias", "user", "seed",
+    "tools", "tool_choice", "response_format",
+    "logprobs", "top_logprobs", "parallel_tool_calls",
+})
+
+# Provider-specific model_params keys that have no OpenAI Chat Completions
+# equivalent and must be silently dropped to keep payloads valid.
+_DROPPED_NON_OPENAI_FIELDS = frozenset({
+    "reasoning_effort",  # Claude CLI / Anthropic-specific
+    "max_depth",         # llm-connect's own depth knob
+    "claude_cli_path",   # adapter wiring leak
+    "json_schema",       # translated below into response_format
+})
+
+
+def _merge_model_params(payload: Dict[str, Any], model_params: Dict[str, Any]) -> None:
+    """Merge RunConfig.model_params into an OpenAI Chat Completions payload.
+
+    Pass-through whitelisted OpenAI keys, translate json_schema into the
+    proper response_format wrapper, drop known provider-specific fields,
+    and ignore anything else rather than letting it through and triggering
+    a 400 from OpenRouter (the failure mode that hit CUST-WP-0045 on
+    2026-06-02 — reasoning_effort and a top-level json_schema were merged
+    into the body and the API rejected both).
+    """
+    schema = model_params.get("json_schema")
+    if schema is not None and "response_format" not in payload:
+        if isinstance(schema, str):
+            try:
+                import json as _json
+                schema = _json.loads(schema)
+            except (ValueError, TypeError):
+                schema = None
+        if isinstance(schema, dict):
+            payload["response_format"] = {
+                "type": "json_schema",
+                "json_schema": {
+                    "name": "structured_output",
+                    "schema": schema,
+                    "strict": True,
+                },
+            }
+
+    for key, value in model_params.items():
+        if key in _DROPPED_NON_OPENAI_FIELDS:
+            continue
+        if key in _OPENAI_PASSTHROUGH_FIELDS:
+            payload[key] = value
+        # else: silently drop unknown keys rather than risk a 400.