feat(api): add qwen/ prefix routing for Alibaba DashScope provider

Users in Discord #clawcode-get-help (web3g) asked for Qwen 3.6 Plus via
native Alibaba DashScope API instead of OpenRouter, which has stricter
rate limits. This commit adds first-class routing for qwen/ and bare
qwen- prefixed model names.

Changes:
- DEFAULT_DASHSCOPE_BASE_URL constant: /compatible-mode/v1 endpoint
- OpenAiCompatConfig::dashscope() factory mirroring openai()/xai()
- DASHSCOPE_ENV_VARS + credential_env_vars() wiring
- metadata_for_model: qwen/ and qwen- prefix routes to DashScope with
  auth_env=DASHSCOPE_API_KEY, reuses ProviderKind::OpenAi because
  DashScope speaks the OpenAI REST shape
- is_reasoning_model: detect qwen-qwq, qwq-*, and *-thinking variants
  so tuning params (temperature, top_p, etc.) get stripped before
  payload assembly (same pattern as o1/o3/grok-3-mini)

Tests added:
- providers::tests::qwen_prefix_routes_to_dashscope_not_anthropic
- openai_compat::tests::qwen_reasoning_variants_are_detected

89 api lib tests passing, 0 failing. cargo fmt --check: clean.

Closes the user-reported gap: 'use Qwen 3.6 Plus via Alibaba API
directly, not OpenRouter' without needing OPENAI_BASE_URL override
or unsetting ANTHROPIC_API_KEY.

rust/crates/api/src/providers/mod.rs

@@ -181,6 +181,19 @@ pub fn metadata_for_model(model: &str) -> Option<ProviderMetadata> {
             default_base_url: openai_compat::DEFAULT_OPENAI_BASE_URL,
         });
     }
+    // Alibaba DashScope compatible-mode endpoint. Routes qwen/* and bare
+    // qwen-* model names (qwen-max, qwen-plus, qwen-turbo, qwen-qwq, etc.)
+    // to the OpenAI-compat client pointed at DashScope's /compatible-mode/v1.
+    // Uses the OpenAi provider kind because DashScope speaks the OpenAI REST
+    // shape — only the base URL and auth env var differ.
+    if canonical.starts_with("qwen/") || canonical.starts_with("qwen-") {
+        return Some(ProviderMetadata {
+            provider: ProviderKind::OpenAi,
+            auth_env: "DASHSCOPE_API_KEY",
+            base_url_env: "DASHSCOPE_BASE_URL",
+            default_base_url: openai_compat::DEFAULT_DASHSCOPE_BASE_URL,
+        });
+    }
     None
 }
 
@@ -386,6 +399,36 @@ mod tests {
         assert_eq!(kind2, ProviderKind::OpenAi);
     }
 
+    #[test]
+    fn qwen_prefix_routes_to_dashscope_not_anthropic() {
+        // User request from Discord #clawcode-get-help: web3g wants to use
+        // Qwen 3.6 Plus via native Alibaba DashScope API (not OpenRouter,
+        // which has lower rate limits). metadata_for_model must route
+        // qwen/* and bare qwen-* to the OpenAi provider kind pointed at
+        // the DashScope compatible-mode endpoint, regardless of whether
+        // ANTHROPIC_API_KEY is present in the environment.
+        let meta = super::metadata_for_model("qwen/qwen-max")
+            .expect("qwen/ prefix must resolve to DashScope metadata");
+        assert_eq!(meta.provider, ProviderKind::OpenAi);
+        assert_eq!(meta.auth_env, "DASHSCOPE_API_KEY");
+        assert_eq!(meta.base_url_env, "DASHSCOPE_BASE_URL");
+        assert!(meta.default_base_url.contains("dashscope.aliyuncs.com"));
+
+        // Bare qwen- prefix also routes
+        let meta2 = super::metadata_for_model("qwen-plus")
+            .expect("qwen- prefix must resolve to DashScope metadata");
+        assert_eq!(meta2.provider, ProviderKind::OpenAi);
+        assert_eq!(meta2.auth_env, "DASHSCOPE_API_KEY");
+
+        // detect_provider_kind must agree even if ANTHROPIC_API_KEY is set
+        let kind = detect_provider_kind("qwen/qwen3-coder");
+        assert_eq!(
+            kind,
+            ProviderKind::OpenAi,
+            "qwen/ prefix must win over auth-sniffer order"
+        );
+    }
+
     #[test]
     fn keeps_existing_max_token_heuristic() {
         assert_eq!(max_tokens_for_model("opus"), 32_000);