fix(env): env var replacement preserve types (#3270)

# What does this PR do?

During env var replacement, we're implicitly converting all config types
to their apparent types (e.g., "true" to True, "123" to 123). This may
be arguably useful for when doing an env var substitution, as those are
always strings, but we should definitely avoid touching config values
that have explicit types and are uninvolved in env var substitution.

## Test Plan

Unit

llama_stack/core/stack.py

@@ -225,7 +225,10 @@ def replace_env_vars(config: Any, path: str = "") -> Any:
 
         try:
             result = re.sub(pattern, get_env_var, config)
+            # Only apply type conversion if substitution actually happened
+            if result != config:
                 return _convert_string_to_proper_type(result)
+            return result
         except EnvVarError as e:
             raise EnvVarError(e.var_name, e.path) from None
 

tests/unit/server/test_replace_env_vars.py

@@ -88,3 +88,10 @@ def test_nested_structures(setup_env_vars):
     }
     expected = {"key1": "test_value", "key2": ["default", "conditional"], "key3": {"nested": None}}
     assert replace_env_vars(data) == expected
+
+
+def test_explicit_strings_preserved(setup_env_vars):
+    # Explicit strings that look like numbers/booleans should remain strings
+    data = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
+    expected = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
+    assert replace_env_vars(data) == expected