fix(opencode-go): cap mimo-v2.5-pro max_tokens at 131072

The opencode-go relay defaults max_tokens to 262144 when none is sent,
but Xiami mimo-v2.5-pro only supports 131072 completion tokens — every
request 400s with "max_tokens is too large: 262144" before the agent
can do anything.

Add a get_max_tokens(model) hook on ProviderProfile (default returns
default_max_tokens) so profiles fronting multiple upstreams can vary
the cap per-model. Wire chat_completions transport through the hook.
Override on OpenCodeGoProfile with mimo-v2.5-pro=131072.

Only mimo-v2.5-pro is capped — other opencode-go models (kimi, glm,
qwen, minimax, other mimo variants) unchanged.

providers/base.py

@@ -129,6 +129,20 @@ class ProviderProfile:
         """
         return {}, {}
 
+    def get_max_tokens(self, model: str | None) -> int | None:
+        """Return the default max_tokens cap for *model*.
+
+        Overrideable hook for providers that need per-model output caps —
+        e.g. a relay that fronts several upstream backends, each with a
+        different completion-token limit. The transport calls this when
+        the user hasn't set an explicit max_tokens.
+
+        Default: return self.default_max_tokens (the static profile field),
+        ignoring the model name. Override in a subclass to vary the cap
+        per-model.
+        """
+        return self.default_max_tokens
+
     def fetch_models(
         self,
         *,