fix(openclaw-plugin): thread recall rerank into search; mark keywordSearch no-op

[Bartok9]

Summary

• @mem0/openclaw-mem0 documented skills.recall.rerank and skills.recall.keywordSearch (defaulted true) but neither was ever threaded into a search call.
• This PR makes rerank actually reach the Mem0 search payload (platform-only) and marks keywordSearch as a documented no-op so the config no longer misleads.

Motivation

Closes #5351.

Users who set rerank: true expecting Mem0 Platform Advanced Retrieval reranking got only the plugin's local rankMemories() + budgetMemories() ordering — the key was inert. keywordSearch was likewise inert because Mem0 v3 removed keyword search from the search API entirely.

Root cause: recall.ts and index.ts buildSearchOptions built searchOpts with only user_id/top_k/threshold/source (comment: "v3.0.0: reranking removed"), and PlatformProvider.search never forwarded rerank to the SDK — even though mem0-ts SearchMemoryOptions still declares rerank?: boolean and camelToSnakeKeys forwards it to POST /v3/memories/search/.

Fix

• rerank (option 1 — thread it through):
  • openclaw/recall.ts — thread recallConfig.rerank into searchOpts.
  • openclaw/index.ts buildSearchOptions — thread recallCfg.rerank (this is the shared builder used by the memory_search tool, so the tool path is fixed too).
  • openclaw/providers.ts — PlatformProvider.search forwards rerank to the SDK; providerToBackend.search passes it through when requested.
  • Platform-only by design: OSSProvider.search selects fields explicitly and safely ignores rerank (the OSS SDK has no reranker).
• keywordSearch (option 2 — mark no-op): v3 removed keyword search from the search API, so there is nothing to thread. Marked @deprecated no-op in types.ts, openclaw.plugin.json schema, and README.md so users aren't misled.

Verification

• cd openclaw && pnpm test (vitest) — added 2 cases in tests/providers.test.ts:
  • forwards rerank to provider.search when opts.rerank is true
  • omits rerank when opts.rerank is falsy
• Existing providerToBackend — search assertions updated for the new (optional) field.
• Type-checked the changed files; rerank?: boolean added to both SearchOptions definitions (types.ts provider-facing and backend/base.ts already had it).

Did NOT change

• OSS-mode search behavior — OSSProvider.search continues to ignore rerank (intentional; no OSS reranker).
• Default semantics for keywordSearch beyond marking it inert/deprecated.