feat: Initial NPM MCP server implementation

- NpmClient with JWT auth and auto-refresh
- FastMCP server with 5 tools (list_proxy_hosts, get_proxy_host_details, get_system_health, search_audit_logs, list_certificates)
- Docker support with multi-stage build using uv
- Supports stdio and streamable HTTP transports
- GitHub Actions for tests and Docker builds

src/npm_mcp/exceptions.py

@@ -0,0 +1,33 @@
+"""Custom exceptions for NPM API client."""
+
+
+class NpmClientError(Exception):
+    """Base exception for NPM client errors."""
+
+    pass
+
+
+class NpmAuthenticationError(NpmClientError):
+    """Raised when authentication fails."""
+
+    pass
+
+
+class NpmConnectionError(NpmClientError):
+    """Raised when connection to NPM fails."""
+
+    pass
+
+
+class NpmNotFoundError(NpmClientError):
+    """Raised when a resource is not found."""
+
+    pass
+
+
+class NpmApiError(NpmClientError):
+    """Raised for general API errors."""
+
+    def __init__(self, message: str, status_code: int | None = None):
+        super().__init__(message)
+        self.status_code = status_code