fix(chunk-context): address PR #767 review — doc_type filter parity + tests

- Add `doc_type` FieldCondition to the chunk_index-path highlighted-image
  Qdrant filter in both `api/visualization.py` and `auth/viz_routes.py`,
  matching the shape of `_get_chunk_by_index_from_qdrant`. Safe today (the
  block is guarded by `doc_type == "file"` and Nextcloud file IDs are
  globally unique) but prevents a latent bug if other doc types start
  storing highlighted images.
- Demote `viz_routes.py` `ValueError` log from `error` to `warning` (lazy
  %-style) — `_parse_int_param` raises on user-supplied bad input, which
  is a 400 not a server error and shouldn't pollute error logs.
- Hoist `effective_chunk_index` to compute once at the top of
  `get_chunk_with_context`, removing two duplicate assignments.
- Add `test_file_doc_type_qdrant_miss_yields_fast_404` to the management
  endpoint tests, locking in the proxy-timeout fix contract.
- Add `tests/unit/test_viz_routes_chunk_context.py` mirroring management
  coverage for the OAuth-session route: param forwarding (chunk_index /
  total_chunks), `doc_type=file` fast 404, and 400 on invalid int params.

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

nextcloud_mcp_server/search/context.py

@@ -271,6 +271,11 @@ async def get_chunk_with_context(
         else (doc_id if isinstance(doc_id, int) else None)
     )
 
+    # Effective chunk_index for adjacent lookups, marker insertion, and the
+    # response payload — keep `chunk_index is not None` distinct from this so
+    # the gate at line ~280 still controls *whether* to take the indexed path.
+    effective_chunk_index = chunk_index if chunk_index is not None else 0
+
     # Try to get chunk from Qdrant (fast path).
     # Prefer chunk_index lookup (always-indexed field) when caller supplied it;
     # fall back to (chunk_start, chunk_end) lookup otherwise.
@@ -296,9 +301,6 @@ async def get_chunk_with_context(
         settings = get_settings()
         chunk_overlap = settings.document_chunk_overlap
 
-        # Effective chunk_index for adjacent lookups and response (default to 0)
-        effective_chunk_index = chunk_index if chunk_index is not None else 0
-
         before_context = ""
         after_context = ""
         has_before_truncation = False
@@ -420,9 +422,6 @@ async def get_chunk_with_context(
     has_before_truncation = context_start > 0
     has_after_truncation = context_end < len(full_text)
 
-    # Effective chunk_index for response (default to 0 when caller didn't supply)
-    effective_chunk_index = chunk_index if chunk_index is not None else 0
-
     # Create marked text with position markers
     marked_text = _insert_position_markers(
         before_context=before_context,