test: add note export round-trip tests

[enlorik]

Supersedes PR #36 because the original branch may contain stale stacked history.

Added test/note_export_round_trip_test.dart with 10 focused tests verifying that generated note content survives the full export cycle. Tests cover Note.toJson/fromJson preserving HTML metadata comments verbatim, sequential append preserving both the first save's and second save's metadata blocks, source URLs surviving through saveAsNewNote and JSON serialization, and confirming no API keys or secret-like strings appear in exported content. Also checks that double-dash sanitization keeps the HTML comment block valid.

Changed files

• test/note_export_round_trip_test.dart (new)

Tests run

• flutter analyze — no issues
• flutter test — 229 passed (was 219 before; 10 new tests added, zero regressions)

Risks / follow-up

• No production code changed; risk is near zero.

Intentionally not changed

• Production export behavior (note_draft_review_provider.dart)
• Persistence architecture or database layer
• Evidence generation, markdown generation format
• Brave settings/provider code
• SearchScreen wiring

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.
To continue using code reviews, you can upgrade your account or add credits to your account and enable them for code reviews in your settings.