fix: repair OpenCode model routing and selection (#4508)

OpenCode Zen and Go are mixed-API-surface providers — different models
behind them use different API surfaces (GPT on Zen uses codex_responses,
Claude on Zen uses anthropic_messages, MiniMax on Go uses
anthropic_messages, GLM/Kimi on Go use chat_completions).

Changes:
- Add normalize_opencode_model_id() and opencode_model_api_mode() to
  models.py for model ID normalization and API surface routing
- Add _provider_supports_explicit_api_mode() to runtime_provider.py
  to prevent stale api_mode from leaking across provider switches
- Wire opencode routing into all three api_mode resolution paths:
  pool entry, api_key provider, and explicit runtime
- Add api_mode field to ModelSwitchResult for propagation through the
  switch pipeline
- Consolidate _PROVIDER_MODELS from main.py into models.py (single
  source of truth, eliminates duplicate dict)
- Add opencode normalization to setup wizard and model picker flows
- Add opencode block to _normalize_model_for_provider in CLI
- Add opencode-zen/go fallback model lists to setup.py

Tests: 160 targeted tests pass (26 new tests covering normalization,
api_mode routing per provider/model, persistence, and setup wizard
normalization).

Based on PR #3017 by SaM13997.

Co-authored-by: SaM13997 <139419381+SaM13997@users.noreply.github.com>

tests/test_codex_models.py

@@ -186,6 +186,22 @@ class TestNormalizeModelForProvider:
         assert changed is True
         assert cli.model == "claude-opus-4.6"
 
+    def test_opencode_go_prefix_stripped(self):
+        cli = _make_cli(model="opencode-go/kimi-k2.5")
+        cli.api_mode = "chat_completions"
+        changed = cli._normalize_model_for_provider("opencode-go")
+        assert changed is True
+        assert cli.model == "kimi-k2.5"
+        assert cli.api_mode == "chat_completions"
+
+    def test_opencode_zen_claude_sets_messages_mode(self):
+        cli = _make_cli(model="opencode-zen/claude-sonnet-4-6")
+        cli.api_mode = "chat_completions"
+        changed = cli._normalize_model_for_provider("opencode-zen")
+        assert changed is True
+        assert cli.model == "claude-sonnet-4-6"
+        assert cli.api_mode == "anthropic_messages"
+
     def test_default_model_replaced(self):
         """No model configured (empty default) gets swapped for codex."""
         import cli as _cli_mod