fix: respect DashScope v1 runtime mode for alibaba (#2459)

Remove the hardcoded Alibaba branch from resolve_runtime_provider()
that forced api_mode='anthropic_messages' regardless of the base URL.

Alibaba now goes through the generic API-key provider path, which
auto-detects the protocol from the URL:
- /apps/anthropic → anthropic_messages (via endswith check)
- /v1 → chat_completions (default)

This fixes Alibaba setup with OpenAI-compatible DashScope endpoints
(e.g. coding-intl.dashscope.aliyuncs.com/v1) that were broken because
runtime always forced Anthropic mode even when setup saved a /v1 URL.

Based on PR #2024 by @kshitijk4poor.

Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>

tests/test_runtime_provider_resolution.py

@@ -534,6 +534,34 @@ def test_minimax_explicit_api_mode_respected(monkeypatch):
     assert resolved["api_mode"] == "chat_completions"
 
 
+def test_alibaba_default_anthropic_endpoint_uses_anthropic_messages(monkeypatch):
+    """Alibaba with default /apps/anthropic URL should use anthropic_messages mode."""
+    monkeypatch.setattr(rp, "resolve_provider", lambda *a, **k: "alibaba")
+    monkeypatch.setattr(rp, "_get_model_config", lambda: {})
+    monkeypatch.setenv("DASHSCOPE_API_KEY", "test-dashscope-key")
+    monkeypatch.delenv("DASHSCOPE_BASE_URL", raising=False)
+
+    resolved = rp.resolve_runtime_provider(requested="alibaba")
+
+    assert resolved["provider"] == "alibaba"
+    assert resolved["api_mode"] == "anthropic_messages"
+    assert resolved["base_url"] == "https://dashscope-intl.aliyuncs.com/apps/anthropic"
+
+
+def test_alibaba_openai_compatible_v1_endpoint_stays_chat_completions(monkeypatch):
+    """Alibaba with /v1 coding endpoint should use chat_completions mode."""
+    monkeypatch.setattr(rp, "resolve_provider", lambda *a, **k: "alibaba")
+    monkeypatch.setattr(rp, "_get_model_config", lambda: {})
+    monkeypatch.setenv("DASHSCOPE_API_KEY", "test-dashscope-key")
+    monkeypatch.setenv("DASHSCOPE_BASE_URL", "https://coding-intl.dashscope.aliyuncs.com/v1")
+
+    resolved = rp.resolve_runtime_provider(requested="alibaba")
+
+    assert resolved["provider"] == "alibaba"
+    assert resolved["api_mode"] == "chat_completions"
+    assert resolved["base_url"] == "https://coding-intl.dashscope.aliyuncs.com/v1"
+
+
 def test_named_custom_provider_anthropic_api_mode(monkeypatch):
     """Custom providers should accept api_mode: anthropic_messages."""
     monkeypatch.setattr(rp, "resolve_provider", lambda *a, **k: "my-anthropic-proxy")