Plain Redis Fallback for `langgraph-redis` (Planned PR)

[fullzer4]

TL;DR: Add a lightweight fallback saver that works on Redis without RediSearch / RedisJSON (e.g., AWS ElastiCache standard, Upstash, GCP Memorystore, older OSS). Uses only core commands; no infra changes for users.

Problem

AsyncShallowRedisSaver depends on RediSearch/RedisJSON via redisvl. On providers/environments without those modules (ElastiCache standard, Upstash w/out FT, Memorystore, older OSS <7.1), initialization fails during module capability checks.

Proposal

Introduce PlainAsyncShallowRedisSaver, a fallback using only core Redis primitives:

• Checkpoints/offsets: HGET/HSET, ZADD, GET/SET
• Per-thread lock: SET key value NX PX <ttl> + release via EVAL (compare-and-del)

These commands are long-standing (e.g., EVAL since 2.6; SET NX/PX in the SET spec) and require no external modules.

Detection & Usage

Extend from_conn_string(...):

1. Try the current module-based saver.
2. If Search/JSON are absent:
   • If ?fallback=plain or allow_plain=True → use Plain variant.
   • Otherwise: auto-fallback with a warning (configurable).

Provider Compatibility (Quick)

Environment	Modules Available	Current Result	With Fallback
AWS ElastiCache (standard)	None	Fails	Works
Upstash (JSON only, no FT)	Partial (no RediSearch)	Fails	Works
Azure Cache (Enterprise tier)	Full (Search + JSON)	Works	Unchanged
GCP Memorystore (standard)	None	Fails	Works
Recent OSS w/ modules installed	Full	Works	Unchanged
Older OSS / no modules (<7.1)	None	Fails	Works

Ask

Would maintainers be interested in this contribution? If so, I’d be happy to open the PR with the fallback implementation, minimal tests, and brief docs (factory flag + small compatibility table). If you prefer an explicit-flag-only approach (no auto-fallback) or different naming than PlainAsyncShallowRedisSaver, please advise and I’ll align the PR accordingly.