fix(models): read OpenRouter live context_length before hardcoded catch-all (#42986)

OpenRouter-routed slugs that are absent from models.dev (e.g. a freshly
shipped anthropic/claude-fable-5) fell through to the generic
DEFAULT_CONTEXT_LENGTHS["claude"]=200K entry and under-reported their real
1M window. The step-6 OpenRouter live-metadata fallback was gated on
`not effective_provider`, but an OpenRouter selection sets
effective_provider="openrouter" (inferred from the base URL), so that
branch was dead code for every OR model.

Add a dedicated step-5 OpenRouter branch that consults the live /models
catalog (authoritative, refreshes as new slugs ship) before models.dev and
the hardcoded family defaults — mirroring the existing Nous/Copilot/GMI
branches. Keeps the Kimi-family 32k underreport guard. Per-model values are
respected (claude-haiku-4.5 stays 200K), so it does not blanket-bump to 1M.

Regression tests cover the fable-5 case, the genuinely-200k case, and the
Kimi guard.

agent/model_metadata.py

@@ -1810,6 +1810,28 @@ def get_model_context_length(
         if ctx is not None:
             save_context_length(model, base_url, ctx)
             return ctx
+    # 5f. OpenRouter live /models metadata — authoritative for OpenRouter-routed
+    # models. OpenRouter's catalog carries per-model context_length (e.g.
+    # anthropic/claude-fable-5 -> 1M) and refreshes as new slugs ship, so it
+    # must win over both models.dev (step 5g) and the hardcoded family catch-all
+    # (step 8). Before this branch, an OpenRouter selection set
+    # effective_provider="openrouter", which (a) made the models.dev lookup miss
+    # brand-new slugs and (b) skipped the step-6 OR fallback (gated on `not
+    # effective_provider`), so a fresh slug like claude-fable-5 fell through to
+    # the generic "claude": 200K entry and under-reported a 1M window. Mirrors
+    # the dedicated Nous/Copilot/GMI branches above.
+    if effective_provider == "openrouter":
+        metadata = fetch_model_metadata()
+        entry = metadata.get(model)
+        if entry:
+            or_ctx = entry.get("context_length")
+            # Guard against the known OpenRouter Kimi-family 32k underreport
+            # (same class the hardcoded overrides exist to mitigate).
+            if isinstance(or_ctx, int) and or_ctx > 0 and not (
+                or_ctx == 32768 and _model_name_suggests_kimi(model)
+            ):
+                return or_ctx
+
     if effective_provider:
         from agent.models_dev import lookup_models_dev_context
         ctx = lookup_models_dev_context(effective_provider, model)