fix(chunk-context): address PR #767 review — drop dead PDF branch, redundant alias, add boundary tests

- Remove unreachable doc_type=="file" branch (and pymupdf/pymupdf4llm
  imports) from _fetch_document_text in search/context.py — the file
  path is short-circuited in get_chunk_with_context before reaching it.
- Drop the redundant `username = request.user.display_name` alias in
  auth/viz_routes.py; both Qdrant scroll filters now reference user_id
  consistently with the rest of the handler.
- Add TestAdjacentChunkBoundary in tests/unit/test_chunk_context_offset_gate.py
  covering chunk_index=0 (before-fetch gate closed) and
  chunk_index=total_chunks-1 (after-fetch gate closed) — the two
  off-by-one boundaries previously untested.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/viz_routes.py

@@ -632,7 +632,6 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
             try:
                 settings = get_settings()
                 qdrant_client = await get_qdrant_client()
-                username = request.user.display_name
 
                 # Prefer chunk_index for the chunk-bbox lookup (always indexed);
                 # fall back to (chunk_start_offset, chunk_end_offset) when not provided.
@@ -646,7 +645,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
                                     key="doc_id", match=MatchValue(value=doc_id_int)
                                 ),
                                 FieldCondition(
-                                    key="user_id", match=MatchValue(value=username)
+                                    key="user_id", match=MatchValue(value=user_id)
                                 ),
                                 FieldCondition(
                                     key="chunk_index",
@@ -674,7 +673,7 @@ async def chunk_context_endpoint(request: Request) -> JSONResponse:
                                     key="doc_id", match=MatchValue(value=doc_id_int)
                                 ),
                                 FieldCondition(
-                                    key="user_id", match=MatchValue(value=username)
+                                    key="user_id", match=MatchValue(value=user_id)
                                 ),
                                 FieldCondition(
                                     key="chunk_start_offset",