fix(document): fix test assertion structure + add entity graph decision comments

- Refactor DocumentLazyLoadingTest: pull value assertions (assertThat) out
  of assertThatCode lambdas so failures surface as AssertionError rather
  than "unexpected exception: AssertionError" (review item 1)
- Add @EntityGraph("Document.full") to findBySenderId, findByReceiversId,
  findConversation, and findSinglePersonCorrespondence — all return full
  Documents to the controller for JSON serialization (review item 2)
- Add "// Callers access only ..." comments to un-graphed methods where no
  lazy associations are touched: findByTags_Id, findByStatus,
  findByMetadataCompleteFalse(Sort), findByMetadataCompleteFalse(Pageable)
- Remove "what" inline comments from @Transactional(readOnly=true)
  on getRecentActivity and getDocumentById — the why is in ADR-022 (item 4)
- Add named-graph coupling consequence to ADR-022: Document.java and
  DocumentRepository.java graph name strings must stay in sync (item 5)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/adr/022-eager-to-lazy-fetch-strategy.md

@@ -102,3 +102,9 @@ open long enough for the caller to use the result."
   `Person` and `Tag` carry this annotation to prevent Jackson from attempting to serialize
   Hibernate proxy internals when the association is not initialized. Any new entity that is
   used as a lazy association and serialized directly (without a DTO) needs the same annotation.
+- **Named graph strings in `Document.java` and `DocumentRepository.java` must stay in sync.**
+  The `@NamedEntityGraph(name = "Document.full")` / `@NamedEntityGraph(name = "Document.list")`
+  definitions on `Document` are referenced by string in every `@EntityGraph(value = "...")` on
+  `DocumentRepository`. If the names diverge (e.g. a graph is renamed in one place but not the
+  other), Spring Data throws at application startup. Always update both files together when
+  renaming or restructuring a named graph.