chore: consolidate property lists, mark unused methods, document validator

Three cleanup items from the review:

#5: _ARTIFACT_PATH_PROPERTIES in template.py was a hand-maintained set
of property names that duplicated PACKAGEABLE_RESOURCE_ARTIFACT_PROPERTIES
in models.py. Replaced with a frozenset derived from the canonical source
so the two cannot drift.

#6: SamTranslatorWrapper.get_original_template() and
get_dynamic_artifact_properties() are only called in tests — callers
now consume LanguageExtensionResult directly. Added a TODO comment
rather than deleting (would require removing ~20 test methods across
3 files — not worth the churn in this PR).

#7: Documented that get_translated_template_if_valid mutates
self.sam_template when language extensions are present. The mutation
is idempotent (re-expanding an already-expanded template is a no-op)
so double-calling is safe, but the comment makes the contract explicit.

Author: bnusunny

samcli/commands/_utils/template.py

@@ -11,6 +11,7 @@
 from botocore.utils import set_value_from_jmespath
 
 from samcli.commands.exceptions import UserException
+from samcli.lib.cfn_language_extensions.models import PACKAGEABLE_RESOURCE_ARTIFACT_PROPERTIES
 from samcli.lib.cfn_language_extensions.utils import (
     FOREACH_REQUIRED_ELEMENTS,
     is_foreach_key,
@@ -29,20 +30,12 @@
 )
 from samcli.yamlhelper import yaml_dump, yaml_parse
 
-# Artifact path property names that represent local file paths in SAM-generated Mappings.
-# These need relative path adjustment when templates are moved between directories.
-_ARTIFACT_PATH_PROPERTIES = {
-    "CodeUri",
-    "ImageUri",
-    "ContentUri",
-    "Content",
-    "DefinitionUri",
-    "BodyS3Location",
-    "DefinitionS3Location",
-    "SchemaUri",
-    "TemplateURL",
-    "Location",
-}
+# Artifact path property names derived from PACKAGEABLE_RESOURCE_ARTIFACT_PROPERTIES
+# so the two lists cannot drift. Used by _update_sam_mappings_relative_paths to
+# identify which SAM-generated Mapping values are local paths needing adjustment.
+_ARTIFACT_PATH_PROPERTIES = frozenset(
+    prop for props in PACKAGEABLE_RESOURCE_ARTIFACT_PROPERTIES.values() for prop in props
+)
 
 
 class TemplateNotFoundException(UserException):

samcli/lib/samlib/wrapper.py

@@ -107,6 +107,10 @@ def run_plugins(self, convert_local_uris=True):
     def template(self):
         return copy.deepcopy(self._sam_template)
 
+    # TODO: get_original_template and get_dynamic_artifact_properties are only
+    # called in tests. Callers now consume LanguageExtensionResult directly.
+    # Consider removing these methods and their backing fields
+    # (_original_template, _dynamic_artifact_properties) in a follow-up.
     def get_original_template(self) -> Dict[str, Any]:
         """
         Get the original unexpanded template for CloudFormation deployment.

samcli/lib/translate/sam_template_validator.py

@@ -87,7 +87,10 @@ def get_translated_template_if_valid(self):
             boto_session=self.boto3_session,
         )
 
-        # Process language extensions before validation if AWS::LanguageExtensions transform is present
+        # Process language extensions before validation if AWS::LanguageExtensions transform is present.
+        # Note: this mutates self.sam_template when LE is present. The assignment is idempotent
+        # (expand_language_extensions returns a deepcopy, and re-expanding an already-expanded
+        # template is a no-op since the transform is still present but ForEach is already resolved).
         parameter_values = dict(IntrinsicsSymbolTable.DEFAULT_PSEUDO_PARAM_VALUES)
         parameter_values.update(self.parameter_overrides)
         result = expand_language_extensions(self.sam_template, parameter_values=parameter_values)