Server: Fix unique constraint error when multiple createSharedFolderUserItems are run concurrently

[personalizedrefrigerator]

Summary

This pull request ignores "unique constraint" thrown while adding (user_id, item_id) pairs to user_items while accepting a share. (See comment).

May partially resolve #12075.

Testing

This pull request includes an automated test that runs createSharedFolderUserItems multiple times concurrently (as could happen if the same share is accepted multiple times concurrently).

With this change, the sync fuzzer fails after 22 steps, with conflicts (currently expected, see #12993).

[personalizedrefrigerator]

Suggested change

	if (!options.ignoreAlreadyExists || !isUniqueConstraintError(error)) {
	if (!isUniqueConstraintError(error)) {

Some (almost all?) calls to UserItemModel.add/UserItemModel.addMulti are already wrapped in a try/catch that ignores unique constraint errors (example). It could make sense to remove the option and always ignore unique constraint errors here.

[personalizedrefrigerator]

Closing temporarily until the following in-CI test failure is resolved:

[@joplin/server]: FAIL dist/models/ShareModel.test.js
[@joplin/server]:   ● ShareModel › should be possible to run createSharedFolderUserItems multiple times concurrently
[@joplin/server]: 
[@joplin/server]:     error: insert into "user_items" ("created_time", "item_id", "updated_time", "user_id") values ($1, $2, $3, $4) - current transaction is aborted, commands ignored until end of transaction block
[@joplin/server]: 
[@joplin/server]:       at Parser.parseErrorMessage (node_modules/pg-protocol/src/parser.ts:369:69)
[@joplin/server]:       at Parser.handlePacket (node_modules/pg-protocol/src/parser.ts:187:21)
[@joplin/server]:       at Parser.parse (node_modules/pg-protocol/src/parser.ts:102:30)
[@joplin/server]:       at Socket.<anonymous> (node_modules/pg-protocol/src/index.ts:7:48)

[personalizedrefrigerator]

I'm re-running CI — the most recent failing test seems unrelated to this pull request:

 [@joplin/transcribe]:   ● FileStorage › should remove files that are older than the given date
[@joplin/transcribe]: 
[@joplin/transcribe]:     expect(received).toBe(expected) // Object.is equality
[@joplin/transcribe]: 
[@joplin/transcribe]:     Expected: 2
[@joplin/transcribe]:     Received: 3
[@joplin/transcribe]: 
[@joplin/transcribe]:       52 | 		await fs.removeOldFiles(new Date('2025-03-03 12:00'));
[@joplin/transcribe]:       53 | 		const files = await readdir('images');
[@joplin/transcribe]:     > 54 | 		expect(files.length).toBe(2);
[@joplin/transcribe]:          | 		                     ^
[@joplin/transcribe]:       55 | 		expect(files.includes(mockedFilenames[2])).toBe(true);
[@joplin/transcribe]:       56 |
[@joplin/transcribe]:       57 | 		for (const file of mockedFiles) {
[@joplin/transcribe]: 
[@joplin/transcribe]:       at Object.toBe (src/services/FileStorage.test.ts:54:24)

[personalizedrefrigerator]

This withTransaction has been moved within the for loop to avoid the following error observed while running tests in CI: current transaction is aborted, commands ignored until end of transaction block.