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>

tests/unit/conftest.py

@@ -8,6 +8,21 @@ import pytest
 from tests.fixtures.storage_backend import storage_backend  # noqa: F401
 
 
+@pytest.fixture
+def metric_sample():
+    """Return a callable reading a Prometheus sample value (0.0 if absent).
+
+    Shared across the metric unit tests so the helper isn't duplicated per
+    module.
+    """
+    from prometheus_client import REGISTRY
+
+    def _sample(name: str, labels: dict[str, str]) -> float:
+        return REGISTRY.get_sample_value(name, labels) or 0.0
+
+    return _sample
+
+
 @pytest.fixture(autouse=True)
 def _reload_dynaconf_after_test():
     """Ensure dynaconf cache is clean between tests.