Overhaul of merge conflict tests

[lukaskubanek]

Warning

This PR depends on #411 and #412 and should only be reviewed once those are merged.

Summary

This PR replaces the existing merge conflict tests in MergeConflictTests.swift with a new test suite that covers a broader set of scenarios across multiple dimensions:

• different fields / same fields
• client newer / server newer / equal timestamps
• conflict on send / conflict on fetch
• ordering on send: fetch before retry / retry before fetch

The new tests expose two real bugs in the current merge conflict resolution logic. These are temporarily marked with withKnownIssue, as the main goal of this PR is to document the current behavior and form a baseline for future work on configurable conflict resolution.

Improvements

• A simpler Post table type (standalone, no parent entity required) instead of Reminder, which depends on RemindersList.
• Snapshots assert on container.privateCloudDatabase directly, reducing boilerplate compared to the full container, as the shared database isn’t interesting for these tests.
• Conflict scenarios use absolute timestamps (e.g. t=30, t=60) for client modification instead of relative increments. This is now aligned with how CKRecords are being treated.
• Test steps are now annotated and ordered to match real-world event sequences. This is most notable with the client and server edits, which used to not be ordered chronologically.
• Local database state is now checked for convergence with the CloudKit state after each scenario resolves.

Bugs Identified

The new tests reveal the following issues in the current implementation:

1. The client always wins on same-field conflicts. Even when the server record has a newer modification timestamp, the client value is retained. This violates the “last edit wins” conflict resolution strategy as previously reported in #354.
2. Field removal does not update the server timestamp. When both client and server null out a field, the server-side field timestamp is not updated to the client’s modification time even when the client is newer.

Overview

#	Test Name	Client Edits	Server Edits	Expected Result	Actual Result	Correct?	Replaces
1	differentFieldsChange_conflictOnSend_clientNewer	isPublished @ t=60	title @ t=30	Both merged	Both merged	Yes	serverRecordUpdatedBeforeClientRecord
2	differentFieldsChange_conflictOnSend_serverNewer	isPublished @ t=30	title @ t=60	Both merged	Both merged	Yes	clientRecordUpdatedBeforeServerRecord
3	differentFieldsChange_conflictOnFetch_clientNewer	isPublished @ t=60	title @ t=30	Both merged	Both merged	Yes	serverAndClientEditDifferentFields
4	differentFieldsChange_conflictOnFetch_serverNewer	isPublished @ t=30	title @ t=60	Both merged	Both merged	Yes	New
5	differentNullableFieldsChange_conflictOnFetch_clientNewer	priority @ t=60	dueDate @ t=30	Both merged	Both merged	Yes	mergeWithNullableFields
6	sameFieldChange_conflictOnSend_retryBeforeFetch_clientNewer	title @ t=60	title @ t=30	Client	Client	Yes	New
7	sameFieldChange_conflictOnSend_retryBeforeFetch_serverNewer	title @ t=30	title @ t=60	Server	Client	No	New
8	sameFieldChange_conflictOnSend_fetchBeforeRetry_clientNewer	title @ t=60	title @ t=30	Client	Client	Yes	serverRecordEditedBeforeClientButProcessedAfterClient
9	sameFieldChange_conflictOnSend_fetchBeforeRetry_serverNewer	title @ t=30	title @ t=60	Server	Client	No	New (PR #353)
10	sameFieldChange_conflictOnSend_equalTimestamps	title @ t=60	title @ t=60	Client	Client	Yes	New
11	sameFieldChange_conflictOnFetch_clientNewer	title @ t=60	title @ t=30	Client	Client	Yes	serverRecordEditedAndProcessedBeforeClient
12	sameFieldChange_conflictOnFetch_serverNewer	title @ t=30	title @ t=60	Server	Client	No	serverRecordEditedAfterClientButProcessedBeforeClient
13	sameFieldChangeAndRemoval_conflictOnSend_clientNewer	Nulls body @ t=60	Changes body @ t=30	Client	Client	Yes	New
14	sameFieldChangeAndRemoval_conflictOnSend_serverNewer	Nulls body @ t=30	Changes body @ t=60	Server	Client	No	New
15	sameFieldRemoval_conflictOnSend_clientNewer	Nulls body @ t=60	Nulls body @ t=30	Both nil, body @ t=60	Both nil, body @ t=30	No	New
16	sameFieldRemoval_conflictOnSend_serverNewer	Nulls body @ t=30	Nulls body @ t=60	Both nil, body @ t=60	Both nil, body @ t=60	Yes	New

Next Steps

I have follow-up PRs ready with minimal changes to fix the issues described above. However, PRs #410, #411, and #412 need to be addressed first before those can be submitted.