Enhance substitution handling in ir.actions.report with value_type support

• Refactor converter building functionality for improved maintainability.

• Added substitution_mixin.py with the new SubstitutionMixin abstract model.

• Updated ir.actions.report to inherit from SubstitutionMixin and implement _get_template to return redner_tmpl_id.

• Updated mail.template to inherit from SubstitutionMixin, replacing its simpler action_get_substitutions with the mixin's implementation, and added _get_template.

models/ir_actions_report.py

@@ -112,14 +112,6 @@
             filetype = rec.redner_filetype
             rec.is_redner_native_format = fmt.get_format(filetype).native
 
-    def _get_template(self):
-        """Get template for ir.actions.report."""
-        return self.redner_tmpl_id
-
-    def _get_substitutions(self):
-        """Get substitutions for ir.actions.report."""
-        return self.substitution_ids
-
     def get_report_metadata(self):
         self.ensure_one()
 
@@ -224,3 +216,19 @@
                 attachment_vals["name"],
             )
         return buffer
+
+    # -------------------------------------------------------------------------
+    # MIXIN METHODS
+    # -------------------------------------------------------------------------
+
+    def _get_substitution_field(self):
+        """Return the substitution field name for ir.actions.report."""
+        return "substitution_ids"
+
+    def _get_template(self):
+        """Get template for ir.actions.report."""
+        return self.redner_tmpl_id
+
+    def _get_substitutions(self):
+        """Get substitutions for ir.actions.report."""
+        return self.substitution_ids