fix(export): make CSV null values consistent across export paths

[markankamaransheogarat-sys]

Problem

Fixes #1157. Previously, csvValue in src/utils/resultExport.ts exported null and undefined as an empty field (""), whereas value_to_csv_string in the streaming exporter (src-tauri/src/export/format.rs) emitted the sentinel NULL. As a result, exported CSV representations differed depending on whether file export was triggered through frontend loaded rows or the backend streaming path.

Changes

1. Frontend csvValue Representation (src/utils/resultExport.ts):

   • Updated csvValue to emit NULL when value === null || value === undefined, aligning directly with src-tauri/src/export/format.rs (value_to_csv_string).
   • Retained empty string "" preservation, numbers (e.g. 0), booleans (e.g. false), objects, and quoting rules via csvField.

2. Frontend Test Suite (tests/utils/resultExport.test.ts):

   • Updated existing loaded result rows CSV tests to assert NULL for null fields.
   • Added parity test verifying a fixture with null, empty string "", literal "NULL", 0, and false produces "c_null,c_empty,c_literal_null,c_zero,c_false\nNULL,,NULL,0,false".

3. Backend Test Suite (src-tauri/src/export/tests.rs):

   • Added test csv_exports_null_empty_literal_zero_false_consistently asserting the identical fixture representation in the Rust streaming exporter.

Verification

• pnpm typecheck (tsc --noEmit): passed with 0 errors.
• pnpm lint: passed (ESLint and theme token check clean).
• vitest run tests/utils/resultExport.test.ts: all 10 tests passed.
• Parity verified across both frontend and backend streaming export paths.

Closes #1157

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

The frontend csvValue now emits NULL for null/undefined, matching value_to_csv_string in src-tauri/src/export/format.rs. Empty strings, 0, false, and literal "NULL" strings are preserved identically on both paths. The updated and new tests on both sides assert the same fixture output. The only caller of formatResultForExport is src/pages/Editor.tsx, which needs no changes.

Files Reviewed (3 files)

• src/utils/resultExport.ts
• tests/utils/resultExport.test.ts
• src-tauri/src/export/tests.rs

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_{Review guidance: REVIEW.md from base branch main}