test(lsp): migrate client tests to effect fixtures

[kitlangton]

Summary

• migrate LSP client tests to the Effect test harness with scoped instance fixtures
• use AppFileSystem for test file writes
• keep subprocess and timing interactions on live Effect tests

Verification

• bunx prettier --write packages/opencode/test/lsp/client.test.ts
• bunx oxlint packages/opencode/test/lsp/client.test.ts
• bun typecheck (packages/opencode)
• bun test --timeout 5000 test/lsp/client.test.ts (packages/opencode)

Notes

• Test names were compared before/after and are unchanged.