US-024: Add token limit metadata for kimi models

Add ModelTokenLimit entries for kimi-k2.5 and kimi-k1.5 to enable
preflight context window validation. Per Moonshot AI documentation:
- Context window: 256,000 tokens
- Max output: 16,384 tokens

Includes 3 unit tests:
- returns_context_window_metadata_for_kimi_models
- kimi_alias_resolves_to_kimi_k25_token_limits
- preflight_blocks_oversized_requests_for_kimi_models

All tests pass, clippy clean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

@@ -289,6 +289,12 @@ pub fn model_token_limit(model: &str) -> Option<ModelTokenLimit> {
             max_output_tokens: 64_000,
             context_window_tokens: 131_072,
         }),
+        // Kimi models via DashScope (Moonshot AI)
+        // Source: https://platform.moonshot.cn/docs/intro
+        "kimi-k2.5" | "kimi-k1.5" => Some(ModelTokenLimit {
+            max_output_tokens: 16_384,
+            context_window_tokens: 256_000,
+        }),
         _ => None,
     }
 }
@@ -744,6 +750,69 @@ mod tests {
             .expect("models without context metadata should skip the guarded preflight");
     }
 
+    #[test]
+    fn returns_context_window_metadata_for_kimi_models() {
+        // kimi-k2.5
+        let k25_limit = model_token_limit("kimi-k2.5")
+            .expect("kimi-k2.5 should have token limit metadata");
+        assert_eq!(k25_limit.max_output_tokens, 16_384);
+        assert_eq!(k25_limit.context_window_tokens, 256_000);
+
+        // kimi-k1.5
+        let k15_limit = model_token_limit("kimi-k1.5")
+            .expect("kimi-k1.5 should have token limit metadata");
+        assert_eq!(k15_limit.max_output_tokens, 16_384);
+        assert_eq!(k15_limit.context_window_tokens, 256_000);
+    }
+
+    #[test]
+    fn kimi_alias_resolves_to_kimi_k25_token_limits() {
+        // The "kimi" alias resolves to "kimi-k2.5" via resolve_model_alias()
+        let alias_limit = model_token_limit("kimi")
+            .expect("kimi alias should resolve to kimi-k2.5 limits");
+        let direct_limit = model_token_limit("kimi-k2.5")
+            .expect("kimi-k2.5 should have limits");
+        assert_eq!(alias_limit.max_output_tokens, direct_limit.max_output_tokens);
+        assert_eq!(
+            alias_limit.context_window_tokens,
+            direct_limit.context_window_tokens
+        );
+    }
+
+    #[test]
+    fn preflight_blocks_oversized_requests_for_kimi_models() {
+        let request = MessageRequest {
+            model: "kimi-k2.5".to_string(),
+            max_tokens: 16_384,
+            messages: vec![InputMessage {
+                role: "user".to_string(),
+                content: vec![InputContentBlock::Text {
+                    text: "x".repeat(1_000_000), // Large input to exceed context window
+                }],
+            }],
+            system: Some("Keep the answer short.".to_string()),
+            tools: None,
+            tool_choice: None,
+            stream: true,
+            ..Default::default()
+        };
+
+        let error = preflight_message_request(&request)
+            .expect_err("oversized request should be rejected for kimi models");
+
+        match error {
+            ApiError::ContextWindowExceeded {
+                model,
+                context_window_tokens,
+                ..
+            } => {
+                assert_eq!(model, "kimi-k2.5");
+                assert_eq!(context_window_tokens, 256_000);
+            }
+            other => panic!("expected context-window preflight failure, got {other:?}"),
+        }
+    }
+
     #[test]
     fn parse_dotenv_extracts_keys_handles_comments_quotes_and_export_prefix() {
         // given