fix(observability): address PR review + SonarCloud findings

Reviewer findings:
- Fix double-count of exhausted-retry failures: the inner final-retry branch
  and the outer except both recorded a processing error. Consolidate to the
  outer handler (single call site); inner branch keeps only the Qdrant-upsert
  error metric. Regression test added.
- Deletes are no longer counted as indexing events: the delete success path
  drops doc_type so astrolabe_documents_indexed_total is not inflated.
  Regression test added.
- Reuse the already-resolved `settings` in _index_document instead of a second
  get_settings() call.
- Use explicit `> 0` guards in record_document_parse / record_embedding instead
  of truthiness checks.

SonarCloud:
- S1244 (BUG): replace float `==` equality in metric tests with pytest.approx.
- S5332 (hotspot): use https in the gateway-URL test fixture.
- S1192: extract the repeated "vector_sync.chunk_count" span-attribute literal
  into a module constant.

Review nit: move the duplicated `_sample` test helper into a shared
`metric_sample` fixture in tests/unit/conftest.py.

Refs Deck #175, PR #831.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/observability/metrics.py

@@ -534,15 +534,15 @@ def record_document_parse(
     ).observe(duration)
     document_parse_total.labels(processor=processor, tier=tier, status=status).inc()
     if status == "success":
-        if pages:
+        if pages > 0:
             document_pages_processed_total.labels(processor=processor, tier=tier).inc(
                 pages
             )
-        if chars:
+        if chars > 0:
             document_chars_processed_total.labels(processor=processor, tier=tier).inc(
                 chars
             )
-        if byte_size:
+        if byte_size > 0:
             document_bytes_processed_total.labels(processor=processor, tier=tier).inc(
                 byte_size
             )
@@ -587,9 +587,9 @@ def record_embedding(
     ).observe(duration)
     embedding_requests_total.labels(kind=kind, provider=provider, status=status).inc()
     if status == "success":
-        if chunks:
+        if chunks > 0:
             embedding_chunks_total.labels(kind=kind, provider=provider).inc(chunks)
-        if chars:
+        if chars > 0:
             embedding_chars_total.labels(kind=kind, provider=provider).inc(chars)