fix(ux): Filter existing members from channel user autocomplete

[Copilot]

The "Add Users" dialog showed existing channel members in autocomplete results, allowing duplicate selections. Backend handled duplicates gracefully but provided poor UX.

Changes

UserAutoCompleteMultiple component

• Added exceptions: string[] prop to exclude usernames from API query
• Updated query key to include exceptions for cache consistency

AddUsers component

• Fetches current room members (channels/private groups only, limit 100)
• Extracts and sorts usernames via single reduce operation
• Passes as exceptions to UserAutoCompleteMultiple

Query optimization

• Pre-sorts exceptions in AddUsers for memoization
• Query key expects pre-sorted array to avoid repeated allocations

API test

• Verifies backend handles duplicate invitations without creating duplicate system messages

// Before: all users shown
<UserAutoCompleteMultiple {...field} />

// After: existing members filtered
<UserAutoCompleteMultiple 
  exceptions={existingMemberUsernames}
  {...field} 
/>

Behavior

• Query disabled for unsupported room types (e.g., DMs)
• Graceful degradation: on fetch error, shows all users (backend still prevents duplicates)
• Channels >100 members may show some existing members in autocomplete (API pagination limit)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Added user can be added again in channel</issue_title>
<issue_description>

Description:

When we try adding a user to channel, we just can simply add his name in search bar and add to him in the channel,
Now if we again search his name in add user we can add him again.

Steps to reproduce:

1. Go to add user in channel
2. Add a user to channel
3. Again search his name in the list.

Expected behavior:

If user is already in channel he/she should not be in list to get added

Actual behavior:

We can add the user again !

You can see doodle1 is already in channel and yet we can add him again.

</issue_description>

<agent_instructions>Check if we already have an API test for this; if not, create one and make the fix.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@geekgonecrazy from the screenshot it looks like doodle1 was actually removed from the channel when you are about to insert 🤔

• Fixes Added user can be added again in channel #13837

---

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[dionisio-bot]

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Latest commit: 8c8c9f6

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[cubic-dev-ai]

No issues found across 4 files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.63%. Comparing base (3048477) to head (8c8c9f6).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #38956      +/-   ##
===========================================
- Coverage    70.64%   70.63%   -0.01%     
===========================================
  Files         3189     3189              
  Lines       112715   112718       +3     
  Branches     20363    20392      +29     
===========================================
- Hits         79627    79619       -8     
- Misses       31044    31055      +11     
  Partials      2044     2044              

Flag	Coverage Δ
unit	71.16% <100.00%> (-0.02%)	⬇️

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