docs(src): expand Google-style docstrings for training utils env module (#131)

- expand require_env docstring with Args, Returns, Raises sections
- expand set_env_defaults docstring with Args and preservation semantics
- document empty-string edge case in require_env
- use collections.abc.Mapping for Python 3.11+ alignment
- align docstring spacing with metrics.py reference convention

📝 - Generated by Copilot

Author: WilliamBerryiii

src/training/utils/env.py

@@ -3,20 +3,40 @@
 from __future__ import annotations
 
 import os
-from typing import Mapping
+from collections.abc import Mapping
 
 
 def require_env(name: str, *, error_type: type[Exception] = RuntimeError) -> str:
-    """Return the value of *name* or raise when missing."""
+    """Return the value of an environment variable or raise when missing.
 
+    Both unset and empty-string values are treated as missing.
+
+    Args:
+        name: Environment variable name to look up.
+        error_type: Exception class to raise on missing value.
+
+    Returns:
+        Non-empty string value of the environment variable.
+
+    Raises:
+        error_type: When *name* is unset or empty.
+    """
     value = os.environ.get(name)
     if not value:
-        raise error_type(f"Environment variable {name} is required for Azure ML bootstrap")
+        raise error_type(
+            f"Environment variable {name} is required for Azure ML bootstrap"
+        )
     return value
 
 
 def set_env_defaults(defaults: Mapping[str, str]) -> None:
-    """Set default values for environment variables that may be unset."""
+    """Populate environment variables with defaults for keys not already set.
+
+    Existing values are preserved. Only missing keys receive the
+    provided default via ``os.environ.setdefault``.
 
+    Args:
+        defaults: Mapping of variable names to default string values.
+    """
     for key, default_value in defaults.items():
         os.environ.setdefault(key, default_value)