fix(opencode): derive api_mode from target model, not stale config default (#15106)

/model kimi-k2.6 on opencode-zen (or glm-5.1 on opencode-go) returned OpenCode's
website 404 HTML page when the user's persisted model.default was a Claude or
MiniMax model. The switched-to chat_completions request hit
https://opencode.ai/zen (or /zen/go) with no /v1 suffix.

Root cause: resolve_runtime_provider() computed api_mode from
model_cfg.get('default') instead of the model being requested. With a Claude
default, it resolved api_mode=anthropic_messages, stripped /v1 from base_url
(required for the Anthropic SDK), then switch_model()'s opencode_model_api_mode
override flipped api_mode back to chat_completions without restoring /v1.

Fix: thread an optional target_model kwarg through resolve_runtime_provider
and _resolve_runtime_from_pool_entry. When the caller is performing an explicit
mid-session model switch (i.e. switch_model()), the target model drives both
api_mode selection and the conditional /v1 strip. Other callers (CLI init,
gateway init, cron, ACP, aux client, delegate, account_usage, tui_gateway) pass
nothing and preserve the existing config-default behavior.

Regression tests added in test_model_switch_opencode_anthropic.py use the REAL
resolver (not a mock) to guard the exact Quentin-repro scenario. Existing tests
that mocked resolve_runtime_provider with 'lambda requested:' had their mock
signatures widened to '**kwargs' to accept the new kwarg.

tests/hermes_cli/test_model_switch_custom_providers.py

@@ -69,7 +69,7 @@ def test_switch_model_accepts_explicit_named_custom_provider(monkeypatch):
     """Shared /model switch pipeline should accept --provider for custom_providers."""
     monkeypatch.setattr(
         "hermes_cli.runtime_provider.resolve_runtime_provider",
-        lambda requested: {
+        lambda **kwargs: {
             "api_key": "no-key-required",
             "base_url": "http://127.0.0.1:4141/v1",
             "api_mode": "chat_completions",