refactor: Improve code quality, error handling, and test coverage

- Add file_lock context manager to eliminate duplicate locking patterns
- Add ValidationError, ConfigurationError, CertificateError exceptions
- Improve rollback logic in haproxy_add_servers (track successful ops only)
- Decompose haproxy_add_domain into smaller helper functions
- Consolidate certificate constants (CERTS_DIR, ACME_HOME) to config.py
- Enhance docstrings for internal functions and magic numbers
- Add pytest framework with 48 new tests (269 -> 317 total)
- Increase test coverage from 76% to 86%
  - servers.py: 58% -> 82%
  - certificates.py: 67% -> 86%
  - configuration.py: 69% -> 94%

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

haproxy_mcp/haproxy_client.py

@@ -89,13 +89,45 @@ def haproxy_cmd_checked(command: str) -> str:
 
 
 def _check_response_for_errors(response: str) -> None:
-    """Check HAProxy response for error indicators.
+    """Check HAProxy response for error indicators and raise if found.
+
+    HAProxy Runtime API returns plain text responses. Success responses are
+    typically empty or contain requested data. Error responses contain
+    specific keywords that indicate the command failed.
 
     Args:
-        response: Response string from HAProxy
+        response: Response string from HAProxy Runtime API command.
 
     Raises:
-        HaproxyError: If response contains error indicators
+        HaproxyError: If response contains any error indicator keyword.
+
+    Error Indicators:
+        - "No such": Resource doesn't exist (e.g., backend, server, map entry)
+        - "not found": Similar to "No such", resource lookup failed
+        - "error": General error in command execution
+        - "failed": Operation could not be completed
+        - "invalid": Malformed command or invalid parameter value
+        - "unknown": Unrecognized command or parameter
+
+    Examples:
+        Successful responses (will NOT raise):
+            - "" (empty string for successful set commands)
+            - "1" (map entry ID after successful add)
+            - Server state data (for show commands)
+
+        Error responses (WILL raise HaproxyError):
+            - "No such server." - Server doesn't exist in specified backend
+            - "No such backend." - Backend name not found
+            - "No such map." - Map file not loaded or doesn't exist
+            - "Entry not found." - Map entry lookup failed
+            - "Invalid server state." - Bad state value for set server state
+            - "unknown keyword 'xyz'" - Unrecognized command parameter
+            - "failed to allocate memory" - Resource allocation failure
+            - "'set server' expects <addr>:<port>" - Invalid command syntax
+
+    Note:
+        The check is case-insensitive to catch variations like "Error:",
+        "ERROR:", "error:" etc. that HAProxy may return.
     """
     error_indicators = ["No such", "not found", "error", "failed", "invalid", "unknown"]
     if response: