fix: ignore placeholder API keys for optional tools

[Zavianx]

Summary

• Do not register web_search providers when their env vars still contain env.example-style your-* placeholder values
• Do not register or invoke x_search with a placeholder X_BEARER_TOKEN
• Avoid selecting OpenAI/Gemini memory embeddings from placeholder API keys
• Add regression coverage for optional tool registration and memory embedding provider selection

Testing

• bun test src/tools/registry.test.ts src/memory/embeddings.test.ts
• bun run typecheck
• bun test