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/tools/certificates.py

@@ -7,7 +7,13 @@ from typing import Annotated
 
 from pydantic import Field
 
-from ..config import logger, SUBPROCESS_TIMEOUT
+from ..config import (
+    logger,
+    SUBPROCESS_TIMEOUT,
+    CERTS_DIR,
+    CERTS_DIR_CONTAINER,
+    ACME_HOME,
+)
 from ..validation import validate_domain
 from ..haproxy_client import haproxy_cmd
 from ..file_ops import (
@@ -16,11 +22,8 @@ from ..file_ops import (
     remove_cert_from_config,
 )
 
-# Certificate paths
-ACME_SH = os.path.expanduser("~/.acme.sh/acme.sh")
-ACME_HOME = os.path.expanduser("~/.acme.sh")
-CERTS_DIR = "/opt/haproxy/certs"
-CERTS_DIR_CONTAINER = "/etc/haproxy/certs"
+# acme.sh script path (derived from ACME_HOME)
+ACME_SH = os.path.join(ACME_HOME, "acme.sh")
 
 # Longer timeout for certificate operations (ACME can be slow)
 CERT_TIMEOUT = 120