Repair embedding resilience for oversized and failed requests

[benclawbot]

Summary

• chunk oversized embedding inputs before provider calls, then mean-pool and L2-normalize the chunk vectors
• keep provider failures explicit so degraded stores remain recoverable rather than silently producing bad vectors
• add shared embedding regeneration for REST and MCP
• default regeneration to NULL-only updates, with force=true for provider/model migrations
• add regression coverage for chunking, provider failure, retry recovery, and forced model migration

Closes #58
Closes #60

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