fix: throw SdkError with CapabilityNotSupported code instead of generic Error

Replace the generic `throw new Error()` in `assertCapability()` with
`throw new SdkError(SdkErrorCode.CapabilityNotSupported, ...)` so that
callers can programmatically distinguish unsupported-capability errors
from other failures without fragile message-string matching.

Fixes #430

Author: kai-agent-free

packages/client/src/client/client.ts

@@ -434,7 +434,7 @@ export class Client extends Protocol<ClientContext> {
 
     protected assertCapability(capability: keyof ServerCapabilities, method: string): void {
         if (!this._serverCapabilities?.[capability]) {
-            throw new Error(`Server does not support ${capability} (required for ${method})`);
+            throw new SdkError(SdkErrorCode.CapabilityNotSupported, `Server does not support ${capability} (required for ${method})`);
         }
     }
 

test/integration/test/client/client.test.ts

@@ -418,6 +418,12 @@ test('should respect server capabilities', async () => {
             argument: { name: 'test', value: 'test' }
         })
     ).rejects.toThrow('Server does not support completions');
+
+    // Verify that unsupported capability errors are SdkError with CapabilityNotSupported code
+    await expect(client.listPrompts()).rejects.toThrow(SdkError);
+    await expect(client.listPrompts()).rejects.toMatchObject({
+        code: SdkErrorCode.CapabilityNotSupported
+    });
 });
 
 /***