fix(conversation): preserve attachment file paths

[ivanarifin]

Summary

• Fixes [Bug]: Images pasted via clipboard or provided as file references are never received by the AI model #2780 by preserving the exact uploaded/selected file paths in the conversation file marker sent to the backend.
• Prevents AionUI timestamp cleanup from rewriting attachment paths before the backend reads them, which could make image/binary files unreachable even though the UI showed attachment chips/thumbnails.
• Updates the focused unit test to assert backend-facing attachment paths remain exact.

Validation

• npm exec --prefix /tmp/aionui-tools --package oxlint@1.56.0 -- oxlint packages/desktop/src/renderer/utils/file/messageFiles.ts tests/unit/messageFiles.test.ts ✅
• npm exec --prefix /tmp/aionui-tools --package oxfmt@0.41.0 -- oxfmt --check packages/desktop/src/renderer/utils/file/messageFiles.ts tests/unit/messageFiles.test.ts ✅
• git diff --check ✅
• node scripts/generate-i18n-types.js && node scripts/check-i18n.js ✅ (pre-existing warnings only)
• npm exec --prefix /tmp/aionui-tools --package vitest@4.0.18 -- vitest run tests/unit/messageFiles.test.ts --root /Users/mac/Projects/AionUi ⚠️ could not start because the temporary npm execution environment could not resolve vitest/config from the project config without installed project dependencies.
• npm exec --prefix /tmp/aionui-tools --package typescript@5.8.3 -- tsc --noEmit ⚠️ could not complete because project dependencies are not installed in this environment, producing missing module/type errors across the repo.
• just push -u origin fix/issue-2780-image-attachments ⚠️ could not run because just is not installed; with explicit approval, used git push -u origin fix/issue-2780-image-attachments as fallback.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...es/desktop/src/renderer/utils/file/messageFiles.ts	80.00%	1 Missing ⚠️

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