feat: Redis knowledge backend via hook registry

[esafwan]

feat: Redis knowledge backend (hook-registered, contract-compliant)

Adds Redis (RediSearch) as a knowledge backend, registered through the huf_knowledge_backends hook introduced in #406 — huf dogfoods its own registry. Supersedes #235, which was written against the pre-#280 architecture (hand-rolled 381-line backend, hardcoded registry dict, first-class DocType fields).

Design

• Direct KnowledgeBackend implementation on redisvl (the contract's "direct implementation" path, like sqlite_vec) — not a LlamaIndex adapter. llama-index-vector-stores-redis 0.4.0 pins llama-index-core<0.13 (env needs 0.14.x for chroma/pgvector stores) and redisvl<0.4, which is broken on Python 3.14 (pydantic v1 shim crash), so the wrapper is unusable on Frappe 16.
• Hook registration in huf/hooks.py: huf_knowledge_backends = {"redis": "huf.ai.knowledge.backends.redis_backend.RedisBackend"}. Not a built-in; proves external-app registration works.
• Connection settings via advanced_config schema (redis_host, redis_port, redis_username, redis_password, redis_index_prefix) per the backend contract — no first-class DocType fields.
• Scoping: site_name + knowledge_source tags on metadata, search filters, and deletes — safe on a shared Redis server.
• Embeddings always HUF-side (contract requirement); vectors stored as float32 bytes; cosine similarity, flat index, VectorQuery + Tag filter expressions.

Testing (bench 16_kbreg, Frappe 16.27, Python 3.14, Redis 8.8.1)

Test	Status
Registry discovery: redis resolved via hook alongside built-ins	✅
get_advanced_config_schema whitelisted API round-trip	✅
Full cycle: text input → Gemini embeddings (3072-dim) → index → semantic search → stats/health → scoped delete	✅
21 mocked unit tests incl. real delete-path assertion	✅ (2 pre-existing standalone-context errors in registry throw-tests, unrelated)
ruff check / ruff format	✅

Dependencies

• Stacked on feat: hook-based knowledge backend registry + backend contract doc (depends on #280) #406 (hook registry) — merge after it.
• pyproject.toml: redisvl>=0.5 (replaces the wrapper).
• Needs a RediSearch-capable Redis (Redis 8+ or redis-stack); Frappe's cache Redis does not qualify.

[esafwan]

Superseded by consolidated vector backend PR #467, now included in integration PR #468.