fix(test): eliminate Desktop smoke false failures

[xuruiray]

Summary

• bind programmatic real-window diagnostics to the stable data-maka-contract="search-modal" contract instead of the removed .maka-search-modal class
• return 204 only when the optional /favicon.ico is absent, while preserving real favicon files and 404 responses for every other missing asset
• add HTTP-level regression coverage for the Storybook static server behavior

Closes #2096

Scope

Test infrastructure only. This PR does not change Desktop product UI, Runtime, IPC, MCP data, credentials, or connection lifecycle.

Current head 10c1c1866 is rebased onto main@121bbe73d. The branch contains one functional commit.

Verification

• npm run format:check — 1,463 files
• npm run lint — 2,373 files
• npm run build
• npm run typecheck
• npm run test:scripts — 105/105
• node --test scripts/storybook-visual-smoke.test.mjs — 1/1
• npm --workspace @maka/desktop test — 1,586/1,586
• npm --workspace @maka/desktop run smoke:programmatic-window — 7/7 on the exact current head
• npm --workspace @maka/desktop run build-storybook
• full Storybook smoke with local system Chrome — 68 manifest checks and 73 catalog renders

GitHub CI on the exact current head is green: all 11 checks passed, including workspace tests, Runtime Host, Headless, Storybook, both E2E shards, aggregate E2E, typecheck, and alignment audit.

Regression behavior

The real Electron probe now confirms that the search modal is present, open, and owns focus through its stable contract. The Storybook server test confirms that only an absent optional favicon receives 204; an existing favicon is served normally and every other missing asset remains 404.

An earlier CI run exposed an unrelated Runtime concurrency flake in an unmodified filesystem-authority test. That exact test passed 11/11 in isolation, and the complete second CI run passed without any Runtime changes in this PR.

[jackwener]

Review by maka-审美专家 — 通过：诊断选择器从被删的 .maka-search-modal 根类迁到稳定契约 data-maka-contract="search-modal"（已核 search-modal.tsx:210 存在）；favicon 204 收窄到「文件确实缺失」路径且路径穿越检查保留；新增 HTTP 层回归测试覆盖 200/204/404 三态；旧字段名 searchModalBackdropPresent 全仓无残留。CI 11/11 绿。合入。