refactor(semantic-search): use ai sdk embedder contract (#70)

## Summary

Simplifies the semantic-search vector embedding API to a single AI
SDK-backed `makeEmbedder` contract. Hosts now create provider models and
pass an explicit embedder into the vector backend.

## Changes

- Replace Gemini, Cloudflare Workers AI, and raw OpenAI-compatible
plugin constructors with generic `makeEmbedder`.
- Remove provider-specific vector backend options such as `workersAi`,
`geminiApiKey`, and embedder model config.
- Keep provider construction only in the local eval harness and add
focused tests for batching, provider options, and dimension validation.

## Tests

- `./node_modules/.bin/oxfmt --check
packages/plugins/semantic-search/src/sdk/embedder.ts
packages/plugins/semantic-search/src/sdk/embedder.test.ts
packages/plugins/semantic-search/src/sdk/embedding-service.ts
packages/plugins/semantic-search/src/sdk/eval.ts
packages/plugins/semantic-search/src/sdk/index.ts
packages/plugins/semantic-search/src/sdk/tool-search-backend.ts
packages/plugins/semantic-search/package.json
packages/plugins/semantic-search/tsup.config.ts`
- `./node_modules/.bin/oxlint -c .oxlintrc.jsonc
packages/plugins/semantic-search/src/sdk/embedder.ts
packages/plugins/semantic-search/src/sdk/embedder.test.ts
packages/plugins/semantic-search/src/sdk/embedding-service.ts
packages/plugins/semantic-search/src/sdk/eval.ts
packages/plugins/semantic-search/src/sdk/index.ts
packages/plugins/semantic-search/src/sdk/tool-search-backend.ts
--deny-warnings`
- `bun run --filter @executor-js/plugin-semantic-search test --
src/sdk/embedder.test.ts src/sdk/plugin.test.ts src/sdk/search.test.ts`
- `bun run --filter @executor-js/plugin-semantic-search typecheck`
- `bun run --filter @executor-js/plugin-semantic-search test`

<!-- stack:links:start -->
### [Stack](https://github.com/aryasaatvik/stack)

1. #69
2. **#70** 👈 current
<!-- stack:links:end -->

Author: aryasaatvik

bun.lock

@@ -17,11 +17,12 @@
     "eval": "bun run src/sdk/eval.ts"
   },
   "dependencies": {
-    "@ai-sdk/google": "^3.0.80",
     "@executor-js/sdk": "workspace:*",
     "ai": "^6.0.191"
   },
   "devDependencies": {
+    "@ai-sdk/google": "^3.0.80",
+    "@ai-sdk/openai-compatible": "^1.0.39",
     "@cloudflare/workers-types": "^4.20250620.0",
     "@effect/atom-react": "catalog:",
     "@effect/vitest": "catalog:",