[WC-3334] Fix DG2 export type bugs

[samuelreichert]

Pull request type

Refactoring (e.g. file rename, variable rename, etc.)
Bug fix (non-breaking change which fixes an issue)

Description

Fix multiple data export bugs in DataGrid2's Excel export feature (WC-3334).

Problems

1. customContent columns ignore exportType — exportValue always exports as text regardless of exportType setting, making numeric values unusable for calculations in Excel
2. Date exports leak time components — Date-only formats (e.g., dd-MMM-yyyy) carry a hidden time component in the cell value, affecting sorting and calculations
3. Boolean exports show TRUE/FALSE — Excel boolean cells always render as TRUE/FALSE regardless of the w field; DataGrid2 displays Yes/No in the UI
4. Long numbers lose precision — Big.toNumber() loses precision for values >15 significant digits, causing scientific notation in Excel

Changes

• Extracted cell-creation logic from DSExportRequest.ts into standalone cell-readers.ts module (pure functions, zero behavior change)
• customContent reader now branches on exportType: parses to Number() for "number", new Date() for "date", with graceful string
  fallback on parse failure
• Added hasTimeComponent(format) / stripTime(date) helpers — when format has no h/s tokens, date values are normalized to midnight UTC
• excelBoolean now returns { t: "s", v: "Yes" | "No" } instead of { t: "b" } to match grid display
• Big values with >15 significant digits export as strings via toFixed() to preserve all digits
• 28 new unit tests covering all reader paths, edge cases, and fallback behavior

[github-actions]

AI Code Review

🔶 Changes requested — one or more medium-severity items must be addressed

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What was reviewed

File	Change
packages/pluggableWidgets/datagrid-web/src/features/data-export/cell-readers.ts	New module: extracted cell-reader logic from DSExportRequest, added customContent type-branching, boolean→Yes/No, date time-stripping, Big precision guard
packages/pluggableWidgets/datagrid-web/src/features/data-export/DSExportRequest.ts	Removed inline reader logic, imports readChunk from new module
packages/pluggableWidgets/datagrid-web/src/features/data-export/__tests__/cell-readers.spec.ts	New test file: 28 tests covering reader paths, fallbacks, precision, and time-stripping
packages/pluggableWidgets/datagrid-web/CHANGELOG.md	Added four ### Fixed entries under [Unreleased]

Skipped (out of scope): dist/, pnpm-lock.yaml

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Findings

🔶 Medium — excelDate type signature allows {t:"d", v: string} — invalid SheetJS cell

File: packages/pluggableWidgets/datagrid-web/src/features/data-export/cell-readers.ts line 71
Problem: excelDate accepts value: string | Date and format?: string. When a caller passes a string value with a non-undefined format, the function returns { t: "d", v: "<string>" } — an invalid SheetJS cell (SheetJS requires t:"d" cells to have a Date as v). The current callers happen not to hit this combination, but the type signature doesn't prevent it. A future caller could easily produce a corrupt export.
Fix: Overload or split the function so t:"d" is only produced when v is a Date:

export function excelDate(value: string): ExcelCell;
export function excelDate(value: Date, format: string): ExcelCell;
export function excelDate(value: string | Date, format?: string): ExcelCell {
    return {
        t: format === undefined || typeof value === "string" ? "s" : "d",
        v: value,
        z: format
    };
}

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🔶 Medium — dynamicText reader computes format but never uses it for numeric/date coercion

File: packages/pluggableWidgets/datagrid-web/src/features/data-export/cell-readers.ts line 142
Problem: The dynamicText reader calls getCellFormat(...) and passes format to excelString. However, excelString only uses format to set the z field — there is no coercion to t:"n" or t:"d" regardless of exportType. This means a dynamicText column with exportType: "number" still exports as a string cell, which is the bug described in problem #1 of the PR for customContent. The same silent no-op exists here but is untested and undocumented.
Fix: Either apply the same number/date branching as customContent, or explicitly document and test that dynamicText always exports as string (and why). If the intent is string-only, remove the getCellFormat call to avoid confusion.

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⚠️ Low — hasTimeComponent regex misses minute-only formats like "mm:ss"

File: packages/pluggableWidgets/datagrid-web/src/features/data-export/cell-readers.ts line 86
Note: /[hs]/i detects h (hours) and s (seconds). A format like "mm:ss" (minutes and seconds, no hours) would match s and correctly preserve time — that path is fine. However, a format of "HH:mm" (hours and minutes, locale variants) would also match H via the case-insensitive flag — also correct. The one edge case not covered by a test is "ss" alone (elapsed seconds format) — worth adding a test case for documentation. No code change required if the current behavior is intentional.

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⚠️ Low — excelString has redundant ?? undefined

File: packages/pluggableWidgets/datagrid-web/src/features/data-export/cell-readers.ts line 67
Note: z: format ?? undefined — since format is already string | undefined, this is a no-op. Simplify to z: format.

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⚠️ Low — Plain number type path in attribute reader is untested

File: packages/pluggableWidgets/datagrid-web/src/features/data-export/__tests__/cell-readers.spec.ts
Note: The attribute reader at line 131 has a branch for typeof value === "number" (native JS number, not Big). All tests use new Big(...). Since Mendix's ListAttributeValue for numeric types always returns Big, this branch is likely dead code — if so, remove it. If it can be reached, add a test with a plain number builder or a comment explaining when it applies.

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Positives

• Clean extraction into a pure-function module (cell-readers.ts) with zero side effects — easy to test and reason about.
• The countSignificantDigits guard for Big precision handles leading zeros, decimals, and negative values correctly.
• stripTime uses Date.UTC consistently, avoiding local-timezone contamination in exported date values.
• Boolean fix (Yes/No string cell instead of {t:"b"}) matches grid display and avoids the Excel TRUE/FALSE rendering issue cleanly.
• Test structure uses the column() factory helper from test-utils correctly, keeping tests concise and avoiding repetition.
• CHANGELOG entries are clear, user-facing, and cover all four fixed bugs.