Fix SS export when image names (without extension) conflict (BL-16498)

[StephenMcConnel]

Fixes the error thrown during Spreadsheet (SS) export when a book contains image files whose names, stripped of extension, collide.

Problem

EPPlus treats worksheet drawing names as case-insensitive and rejects duplicates. Two source files such as image.jpg and image.png (or Foo.png and foo.jpg) reduce to the same drawing name, so worksheet.Drawings.AddPicture(...) threw during export.

Fix

• d34355b92 — make the duplicate-name check case-insensitive and rename a colliding image to {name}-{rowNum}.
• 74e34419d — guarantee the renamed value is actually unique. The first commit appended the -{rowNum} suffix blindly (no re-check) and renamed using the original casing, so the result could still collide (e.g. three images sharing a base name in one row, or two same-row names differing only in case). Now we loop, comparing case-insensitively, until the name is truly unique — matching EPPlus semantics.

Testing

• Full Spreadsheet test suite passes (376 tests).
• BloomExe and BloomTests build cleanly.

Ref: https://issues.bloomlibrary.org/issue/BL-16498

Devin review

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This change is 

[greptile-apps]

Greptile Summary

This PR fixes a crash during spreadsheet export when a book contains image files whose base names (after stripping the extension) collide — including case-insensitive collisions such as conflict.png / Conflict.jpg — by replacing a broken single-attempt, case-sensitive deduplication check with a case-insensitive do...while loop that keeps incrementing the name suffix until EPPlus's uniqueness constraint is satisfied.

• SpreadsheetIO.cs: The embedImage local function now defines a NameIsTaken helper that compares candidate drawing names case-insensitively against all existing worksheet drawings, then loops (do...while) with an incrementing suffix until a genuinely unique name is found — matching EPPlus's own case-insensitive uniqueness semantics.
• SpreadsheetImagesTests.cs: Adds a regression test (Export_ImagesWithConflictingNames_AllEmbeddedWithoutError) that creates three images whose base names collide both by extension and by case, exports the book, re-reads the .xlsx file, and asserts all three images are embedded without error or warning.

Important Files Changed

Filename	Overview
src/BloomExe/Spreadsheet/SpreadsheetIO.cs	Replaces a case-sensitive, single-attempt duplicate-name check with a case-insensitive do...while loop that keeps incrementing the suffix until the drawing name is genuinely unique, matching EPPlus's own case-insensitive uniqueness requirement.
src/BloomTests/Spreadsheet/SpreadsheetImagesTests.cs	Adds a regression test that exports a book with three images whose base names collide both exactly and case-insensitively, then verifies all three embed without error and no warnings are raised.

_{Reviews (2): Last reviewed commit: "Add regression test for SS export image-..." | Re-trigger Greptile}

[StephenMcConnel]

[Claude Opus 4.8 (1M context)] Consulted Devin on 2026-07-07 20:13 UTC up to commit 74e34419d — 0 Bugs, 0 Flags (clean). Greptile also reviewed clean.

[StephenMcConnel]

[Claude Opus 4.8 (1M context)] Consulted Devin on 2026-07-07 20:54 UTC up to commit e3c3d371e — 0 Bugs, 0 Flags. (3 Informational observations only; not blocking.) Greptile also re-reviewed clean.