fix(phase8/task46/h4): graceful skip for default chat collection without embedding model

[earayu]

Summary

H4 (task #46) graceful degradation for provider-independent flows, per architect canonical msg=8b6b4bc3 (Option α) and msg=2c4f4d8c (Option H4-B refined).

Closes the two related bugs surfaced by @weihong #41 H4 inventory (msg=1f04e57a):

Bug 1 — registration flow ValueError noise

ChatCollectionService.create_user_chat_collection raised ValueError("No suitable embedding model found...") on no-provider users; caught (and silently logged as ERROR) by user_manager.py try/except. Replace with logger.info + return None (graceful skip). The single non-registration caller (chat_document_service.upload_chat_document) surfaces an HTTP 400 instead — chat document upload genuinely needs an embedding model.

Bug 2 — smoke Celery NoneType retry storm (root cause)

CollectionTask._initialize_vector_databases unconditionally called get_collection_embedding_service_sync, even for collections with enable_vector=false (smoke 02_collection.hurl, KG-only tenants). When config.embedding is None, base_embedding.py:158 accessed .model_service_provider on NoneType → AttributeError → Celery retry storm.

Add an enable_vector guard symmetric with the existing enable_fulltext guard in _initialize_fulltext_index (one-line fix).

Scope discipline

• aperag/tasks/collection.py 1-line guard = local bug fix, NOT a relocation. Option I scope-lock (no aperag/tasks/ integral migration) still holds — explicitly confirmed by @架构师 msg=1e845856.
• aperag/domains/knowledge_base/service/collection_service.py was originally listed in canonical write set; analysis shows once _initialize_vector_databases is fixed at root, the collection_init_task retry pipeline stays generic (transient errors should still retry). Not modified.

Gate

• uv run pytest tests/unit_test/chat/test_chat_collection_graceful_skip.py tests/unit_test/tasks/test_collection_init_skip.py — 5/5 pass
• uv run pytest tests/unit_test/test_modularization_boundaries.py — 21/21 pass (G1-G19 hard invariants intact)
• uv run ruff format --check — clean
• uv run ruff check — clean
• Existing chat domain tests tests/unit_test/chat/ — 11/11 pass

Operational gates (architect msg=1e845856)

• Registration flow no longer logs No suitable embedding model ERROR
• Smoke compose log no longer emits NoneType.*model_service_provider Celery retry (root cause path closed by _initialize_vector_databases enable_vector guard)
• tests/e2e_http/bootstrap/PROTOCOL.md unchanged (protocol itself was correct)

Test plan

• Unit tests cover graceful skip for both registration and existing-collection paths
• Unit tests cover _initialize_vector_databases short-circuit on enable_vector=false
• Unit tests cover happy path (enable_vector=true with embedding) still resolves embedding service exactly once
• CI lint-and-unit green
• CI e2e-http-smoke no longer shows NoneType.*model_service_provider retry spam

Closes

Task #46 — Phase 8 第二批 H4 graceful skip for default chat collection without embedding model

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