fix(review): _enum_fields validation, gate classify_from_text flags, OCR warnings

Address PR #858 review round 3:

- document_tier1_engine / document_ocr_provider now validate + normalize via
  Settings.__post_init__ _enum_fields (the repo's canonical opt-in-enum pattern;
  case-insensitive) instead of dynaconf Validators. A typo now raises ValueError
  at load and "Gateway" normalizes to "gateway".
- classify_from_text gates no_text_layer/bad_text_layer on ocr_frac >=
  OCR_PAGE_FRACTION, matching classify_pdf -- a "fast"-routed doc with a few junk
  pages no longer emits a misleading flag (keeps the shadow vs hot-path
  classification metrics consistent).
- build_ocr_backend warns when an EXPLICIT provider is misconfigured
  (gateway without EMBEDDING_GATEWAY_URL, mistral without MISTRAL_API_KEY)
  instead of silently returning None.
- Pypdfium2FastProcessor.health_check probes the import; documented why
  OcrProcessor.health_check is unconditionally True (lazy per-tenant backends).
- Removed the leftover per-boundary / per-chunk debug logging loops.

Tests: enum normalization + rejection for the two new settings.

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

tests/unit/test_decomposition_config.py

@@ -29,9 +29,13 @@ class TestDecompositionDefaults:
         s = Settings(
             collection_metadata_source=" QDRANT ",
             mcp_role=" API ",
+            document_tier1_engine=" PyPDFium2 ",
+            document_ocr_provider=" Gateway ",
         )
         assert s.collection_metadata_source == "qdrant"
         assert s.mcp_role == "api"
+        assert s.document_tier1_engine == "pypdfium2"
+        assert s.document_ocr_provider == "gateway"
 
 
 class TestEnumValidation:
@@ -41,6 +45,8 @@ class TestEnumValidation:
             ("embedding_provider", "openai"),
             ("mcp_role", "leader"),
             ("collection_metadata_source", "redis"),
+            ("document_tier1_engine", "mupdf"),
+            ("document_ocr_provider", "gatway"),
         ],
     )
     def test_invalid_enum_rejected(self, field, value):