refactor(tool): use key defaults in parameter schemas

[kitlangton]

Summary

• switch codesearch, webfetch, and websearch defaults to key defaults and finite numbers where that better matches native Effect JSON Schema
• teach the effect-zod bridge to emit defaults for key-defaulted struct fields so LLM-facing schemas stay aligned
• add focused tests for native-schema invariants and key-default bridge behavior

Verification

• bun run test test/util/effect-zod.test.ts test/tool/parameters.test.ts test/tool/tool-define.test.ts
• bun typecheck