fix(opencode): every credential-resolution surface keys off the model it will send; family heal only for built-in providers

Closes the remaining atoms of #112600.

A) The CLI startup path already passes `-m` as target_model (c358a6fba0), but
   its siblings still resolved credentials against config's `default`: the CLI
   auth-fallback rung, `--resume` credential re-resolution, the gateway
   provider-override helper (channel overrides, persisted /model switches,
   API-server provider refresh), the gateway fallback chain, the TUI /model
   switch-from runtime and ACP agent construction. With a `*-free` default the
   OpenCode free-tier rung fired first and a Go-only model was built against
   the keyless Zen relay ("Model mimo-v2.5 is not supported"). Each now passes
   the effective model; `_resolve_runtime_agent_kwargs_for_provider` grows an
   optional `target_model` and the two test stubs of it accept the kwarg.

B) normalize_opencode_base_url rewrote the /zen vs /zen/go segment for ANY
   provider matched by opencode_provider_family, including custom providers
   merely named after a family (`opencode-go-bridge`, #85589) whose relay the
   user declared explicitly in `providers:`. The family heal now applies to the
   built-in canonical providers only; custom prefix-named providers keep their
   per-model api_mode routing and /v1 handling. Documented in the providers
   guide.

C) Same function: the official-host check uses parsed.hostname (a port no
   longer defeats the heal) and only the path is edited, so query/fragment
   round-trip instead of being dropped.

Fixes #112600

acp_adapter/session.py

@@ -400,7 +400,8 @@ class SessionManager:
             "cwd": cwd,
         }
         try:
-            runtime = resolve_runtime_provider(requested=requested_provider or config_provider)
+            runtime = resolve_runtime_provider(
+                requested=requested_provider or config_provider, target_model=(model or default_model) or None)
             kwargs.update({
                 "provider": runtime.get("provider"), "api_mode": api_mode or runtime.get("api_mode"),
                 "base_url": base_url or runtime.get("base_url"), "api_key": runtime.get("api_key"),