feat(attachments): native PDF + audio translators, accept on upload

PDF and audio attachments now work end-to-end on the native provider
lanes; non-native lanes degrade to a placeholder (client-side fallback
lands next). Capability flags are populated but not yet consumed by a
wire-build gate.

- providers: Anthropic PDF -> base64 document; OpenAI Responses PDF ->
  input_file; compat/Google inline_document_parts PDF -> placeholder
  (fixes the base64-as-text mangle); audio = input_audio passthrough on
  the compat lane (omni), defensive text placeholders on Anthropic +
  Responses
- capabilities: supports_pdf on cloud Claude + OpenAI chat models;
  local/default/compat stay False (-> client-side fallback)
- upload: classifier accepts pdf (32 MiB) + audio (25 MiB); endpoint
  multipart read cap raised to PDF_SIZE_CAP
- hygiene: consolidate the duplicated upload classification into one
  attachments.classify_upload (+ UploadRejection); collapse
  AttachmentUploadHelpers to a single classify_upload callable
- tests: PDF/audio translator shapes, capability flags, classify_upload

tests/test_coordinator_endpoints.py

@@ -48,10 +48,7 @@ from turnstone.console.server import (
     coordinator_tasks,
 )
 from turnstone.core.attachments import (
-    classify_text_attachment as _coord_test_classify_text,
-)
-from turnstone.core.attachments import (
-    sniff_image_mime as _coord_test_sniff_image,
+    classify_upload as _coord_test_classify_upload,
 )
 from turnstone.core.auth import AuthResult
 from turnstone.core.session_routes import (
@@ -107,8 +104,7 @@ _coord_endpoint_config = SessionEndpointConfig(
     supports_attachments=True,
     attachment_owner_resolver=_coord_attach_owner,
     attachment_helpers=AttachmentUploadHelpers(
-        sniff_image_mime=_coord_test_sniff_image,
-        classify_text_attachment=_coord_test_classify_text,
+        classify_upload=_coord_test_classify_upload,
     ),
     spawn_metrics=None,
     emit_message_queued=True,