feat(07-03): implement cookie validation functionality

- Add CookieValidationResult Pydantic model with valid, username, user_id, reason fields
- Add validate_cookie async method to ZhihuLogin class
- Validate cookie via Zhihu /api/v4/me endpoint
- Handle 200, 401, and other status codes appropriately
- Handle network errors gracefully

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

kb_workflow/auth/zhihu_login.py

@@ -34,6 +34,21 @@ class ZhihuLoginResult(BaseModel):
     error_message: Optional[str] = Field(None, description="Error message if failed")
 
 
+class CookieValidationResult(BaseModel):
+    """Cookie validation result.
+
+    Attributes:
+        valid: Whether the cookie is valid
+        username: User's display name if valid
+        user_id: User's ID if valid
+        reason: Reason for invalid cookie
+    """
+    valid: bool = Field(..., description="Whether cookie is valid")
+    username: Optional[str] = Field(None, description="User display name")
+    user_id: Optional[str] = Field(None, description="User ID")
+    reason: Optional[str] = Field(None, description="Reason if invalid")
+
+
 class ZhihuLogin:
     """Zhihu QR Code Login Handler.
 
@@ -54,6 +69,7 @@ class ZhihuLogin:
         "canvas[class*='qrcode']",
         "canvas"
     ]
+    VALIDATION_URL = "https://www.zhihu.com/api/v4/me"
 
     def __init__(
         self,
@@ -218,3 +234,43 @@ class ZhihuLogin:
         finally:
             if self.browser:
                 self.browser.close()
+
+    async def validate_cookie(self, cookie_str: str = None) -> CookieValidationResult:
+        """Validate cookie validity.
+
+        Checks cookie by requesting Zhihu user info API.
+
+        Args:
+            cookie_str: Cookie string to validate. If None, uses already loaded cookie.
+
+        Returns:
+            CookieValidationResult with validation status and user info
+        """
+        # If cookie_str provided, need to restart browser with new cookie
+        if cookie_str:
+            if self.browser.browser:
+                self.browser.close()
+            self.browser.start(cookie=cookie_str)
+        elif not self.browser.page:
+            return CookieValidationResult(valid=False, reason="No cookie provided")
+
+        try:
+            # Use Playwright's request API
+            response = await self.browser.page.request.get(self.VALIDATION_URL)
+
+            if response.status == 200:
+                user_info = await response.json()
+                return CookieValidationResult(
+                    valid=True,
+                    username=user_info.get("name", ""),
+                    user_id=str(user_info.get("id", ""))
+                )
+            elif response.status == 401:
+                return CookieValidationResult(valid=False, reason="Cookie expired")
+            else:
+                return CookieValidationResult(
+                    valid=False,
+                    reason=f"Validation failed with status {response.status}"
+                )
+        except Exception as e:
+            return CookieValidationResult(valid=False, reason=str(e))