fix: repair deferred imports to preserve module-level names for test patching (#831)

A previous commit moved SDK imports into __init__/methods and
stashed them on self, which broke @patch targets in 24 unit tests.

This fixes the approach: chunker and pdf_decoder use module-level
sentinels with global/if-None guards so imports are still deferred but
patchable. Google AI Studio reverts to standard module-level imports
since the module is only loaded when communicating with Gemini.
Keeps lazy loading on other imports.

trustgraph-flow/trustgraph/decoding/pdf/pdf_decoder.py

@@ -15,6 +15,9 @@ from ... schema import Document, TextDocument, Metadata
 from ... schema import librarian_request_queue, librarian_response_queue
 from ... schema import Triples
 from ... base import FlowProcessor, ConsumerSpec, ProducerSpec, LibrarianClient
+
+PyPDFLoader = None
+
 from ... provenance import (
     document_uri, page_uri as make_page_uri, derived_entity_triples,
     set_graph, GRAPH_SOURCE,
@@ -128,7 +131,12 @@ class Processor(FlowProcessor):
                 fp.write(base64.b64decode(v.data))
                 fp.close()
 
-            from langchain_community.document_loaders import PyPDFLoader
+            global PyPDFLoader
+            if PyPDFLoader is None:
+                from langchain_community.document_loaders import (
+                    PyPDFLoader as _cls,
+                )
+                PyPDFLoader = _cls
             loader = PyPDFLoader(temp_path)
             pages = loader.load()