Initial commit: FastGPT Python SDK Phase 1

Implement core infrastructure:

- BaseClientMixin with retry logic and validation
- FastGPTClient base class with httpx
- ChatClient with 11 chat operation methods
- AppClient for analytics and logs
- Custom exceptions (APIError, AuthenticationError, etc.)
- Package configuration (pyproject.toml, setup.py)
- Documentation (README.md, CLAUDE.md)
- Basic usage examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

fastgpt_client/exceptions.py

@@ -0,0 +1,43 @@
+"""FastGPT Client Exceptions."""
+
+
+class FastGPTError(Exception):
+    """Base exception for all FastGPT errors."""
+
+    def __init__(self, message: str, status_code: int = None, response_data: dict = None):
+        self.message = message
+        self.status_code = status_code
+        self.response_data = response_data or {}
+        super().__init__(self.message)
+
+
+class APIError(FastGPTError):
+    """General API error (4xx, 5xx responses)."""
+
+    pass
+
+
+class AuthenticationError(FastGPTError):
+    """Authentication failed (401)."""
+
+    pass
+
+
+class RateLimitError(FastGPTError):
+    """Rate limit exceeded (429)."""
+
+    def __init__(self, message: str, retry_after: str = None, status_code: int = None, response_data: dict = None):
+        super().__init__(message, status_code, response_data)
+        self.retry_after = retry_after
+
+
+class ValidationError(FastGPTError):
+    """Invalid request parameters (422)."""
+
+    pass
+
+
+class StreamParseError(FastGPTError):
+    """Error parsing streaming response."""
+
+    pass