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/exceptions.py

@@ -9,3 +9,18 @@ class HaproxyError(Exception):
 class NoAvailablePoolError(HaproxyError):
     """All pool backends are in use."""
     pass
+
+
+class ValidationError(HaproxyError):
+    """Input validation error."""
+    pass
+
+
+class ConfigurationError(HaproxyError):
+    """Configuration file or state error."""
+    pass
+
+
+class CertificateError(HaproxyError):
+    """SSL/TLS certificate operation error."""
+    pass