fix(mcp): pre-compile env-var regex and unify interpolation

Remove redundant inner `import re` and regex recompilation on every call in
_interpolate_env_vars. Add module-level _ENV_VAR_PATTERN compiled once.

Replace the separate _interpolate_value() in mcp_config.py (which used \w+
and would silently fail on env vars containing hyphens or dots) with the
shared _ENV_VAR_PATTERN from mcp_tool.py. Remove now-unused import re.

tools/mcp_tool.py

@@ -279,6 +279,11 @@ _CREDENTIAL_PATTERN = re.compile(
     re.IGNORECASE,
 )
 
+# Pre-compiled pattern for ${VAR_NAME} style env-var interpolation.
+# Supports any non-} characters in the variable name (hyphens, dots, etc.)
+# so providers like MY-VAR or my.var work correctly.
+_ENV_VAR_PATTERN = re.compile(r"\$\{([^}]+)\}")
+
 
 # ---------------------------------------------------------------------------
 # Security helpers
@@ -2104,7 +2109,7 @@ def _interpolate_env_vars(value):
     if isinstance(value, str):
         def _replace(m):
             return os.environ.get(m.group(1), m.group(0))
-        return re.sub(r"\$\{([^}]+)\}", _replace, value)
+        return _ENV_VAR_PATTERN.sub(_replace, value)
     if isinstance(value, dict):
         return {k: _interpolate_env_vars(v) for k, v in value.items()}
     if isinstance(value, list):