feat: govern attachment exceptions and ownership transfers

src/govoplan_campaign/backend/messages/builder.py

@@ -16,6 +16,7 @@ from govoplan_campaign.backend.attachments.resolver import (
     EntryAttachmentResolution,
     MessageAttachmentStatus,
     ResolvedAttachment,
+    effective_send_without_attachments_behavior,
     resolve_entry_attachments,
 )
 from govoplan_campaign.backend.campaign.addressing import effective_address_lists, formatted_recipient
@@ -172,6 +173,11 @@ def _attachment_summaries(resolution: EntryAttachmentResolution) -> list[Message
             label=attachment.label,
             status=attachment.status.value,
             behavior=attachment.behavior.value if attachment.behavior else None,
+            missing_policy=(
+                attachment.missing_policy.model_dump(mode="json")
+                if attachment.missing_policy
+                else None
+            ),
             required=attachment.required,
             allow_multiple=attachment.allow_multiple,
             zip_enabled=attachment.zip_enabled,
@@ -200,6 +206,7 @@ def _message_issues_from_attachment_resolution(resolution: EntryAttachmentResolu
             message=issue.message,
             behavior=issue.behavior.value if issue.behavior else None,
             source="attachments",
+            details=issue.details,
         )
         for issue in resolution.issues
     ]
@@ -219,12 +226,6 @@ def _append_no_attachment_coverage_issue(issues: list[MessageIssue]) -> None:
     )
 
 
-def _send_without_attachments_behavior(config: CampaignConfig) -> Behavior:
-    return config.attachments.send_without_attachments_behavior or (
-        Behavior.CONTINUE if config.attachments.send_without_attachments else Behavior.BLOCK
-    )
-
-
 def _safe_filename(value: str | None, fallback: str) -> str:
     raw = value or fallback
     safe = re.sub(r"[^A-Za-z0-9_.-]+", "_", raw).strip("._")
@@ -761,7 +762,11 @@ def _build_mime_message(
             values=rendered.values,
             work_dir=work_dir,
         )
-        if attachment_count == 0 and context.resolution.attachments and _send_without_attachments_behavior(config) == Behavior.BLOCK:
+        if (
+            attachment_count == 0
+            and context.resolution.attachments
+            and effective_send_without_attachments_behavior(config) == Behavior.BLOCK
+        ):
             _append_no_attachment_coverage_issue(context.issues)
             return _MimeBuildResult(
                 message=None,
@@ -785,6 +790,16 @@ def _build_mime_message(
                 source="attachments",
             )
         )
+    except OSError as exc:
+        context.issues.append(
+            MessageIssue(
+                severity="error",
+                code="attachment_unreadable",
+                message="A resolved attachment could not be read while building the message.",
+                behavior="block",
+                source=f"attachments:{type(exc).__name__}",
+            )
+        )
     except Exception as exc:
         context.issues.append(
             MessageIssue(