Fixes#89691.
Memory search now treats explicitly configured non-local embedding providers as required. When that provider is unavailable, search and sync surface an unavailable memory-search result instead of silently returning FTS-only recall.
Unset/default/local/none-style paths keep FTS fallback so existing workflows do not lose keyword recall entirely. The fallback state is now surfaced in diagnostics/status instead of being hidden.
Maintainer merge note: current CI still has unrelated baseline boundary failures in extensions/google/google.live.test.ts and extensions/minimax/minimax.live.test.ts. This PR does not touch those files; the PR-specific memory, docs, lint, type, security, and ClawSweeper checks were reviewed before merge.
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