Fix unread message counter not accounting for deleted messages

[Copilot]

Problem

When a user deletes messages from a conversation, the unread message counters for other users are not updated to reflect that some of their unread messages have been deleted. This causes incorrect unread counts to be displayed in the UI.

Scenario:

1. User A sends messages 1, 2, 3, 4, 5 to User B
2. User B reads messages 1, 2 (readID = 2, unread count = 3)
3. User A deletes messages 3, 4 (soft delete)
4. User B should now have only 1 unread message (message 5), but the counter still shows 3

This affects both online users and users who come back online later, and applies to both soft deletes and hard deletes.

Root Cause

The handleNoteBroadcast function calculates unread counts using simple arithmetic (pud.readID - msg.Note.SeqId) without considering that some messages in the range may have been deleted. The replyDelMsg function successfully deletes messages but doesn't update unread counters for affected users.

Solution

Added logic to update unread counters when messages are deleted:

1. Helper function calculateUnreadInRanges: Efficiently calculates how many unread messages fall within given deletion ranges using intersection logic. Uses linear search to find the first relevant range (where rangeEnd > readID) and then iterates to sum up unread messages in the deleted ranges.

2. Enhanced replyDelMsg: After successful deletion, iterates through all users with read permissions and decreases their unread count by the number of unread messages that were deleted. This applies to:

   • Hard deletes: Updates unread counters for all users since hard deletes remove messages for everyone
   • Soft deletes: Updates unread counters for other users (not the deleting user)

3. Minimal performance impact: Only runs during message deletion operations (rare events), not during normal read operations.

Technical Details

• Updates unread counters for both hard and soft deletes
• Uses existing usersUpdateUnread infrastructure for consistency
• Preserves all existing functionality and API compatibility
• Zero impact on read notification performance

Testing

• Added comprehensive unit tests for calculateUnreadInRanges covering edge cases
• Added integration test TestReplyDelMsgUpdatesUnreadCounters for soft delete scenario
• Added integration test TestReplyDelMsgHardDelete for hard delete scenario
• All existing tests continue to pass

Example calculation:

// User has readID=5, lastID=10 (unread messages: 6,7,8,9,10)
// Delete range [7,9) removes messages 7,8
// Result: 2 unread messages removed from count
ranges := []types.Range{{Low: 7, Hi: 9}}
deleted := calculateUnreadInRanges(5, 10, ranges) // Returns 2

Fixes #898

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[or-else]

@copilot Please inplement fixes according to code review.

[Copilot]

@copilot Please inplement fixes according to code review.

All fixes implemented in dd3d3c9:

• Removed .gitignore
• Optimized calculateUnreadInRanges with binary search
• Added hard delete test case

[or-else]

@copilot Still weak, but OK, I'll clean it up later.