fix(group): use array-aware middleware on /group/participant

[netoduwe]

POST /group/participant was registered with ValidateJIDFields("number", "participants"), which only handles single-string fields. Because participants is an array, the type assertion fails and every request is rejected with 400 "participants is required and cannot be empty" before reaching the UpdateParticipant handler — breaking all participant actions (add / remove / promote / demote).

This swaps to ValidateMultipleNumbers("participants") — the same array-aware middleware already used by /group/create for the same field — and drops the unused "number" argument.

Reproduced on v0.6.1, 0.7.0, 0.7.1 and current main.

Related Issue

Closes #97

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Summary by Sourcery

Bug Fixes:

• Correct validation middleware for POST /group/participant to accept an array of participants instead of rejecting all requests.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR fixes participant-related group actions by switching the /group/participant route to use the array-aware JID validation middleware consistent with /group/create, ensuring the participants array is validated correctly instead of being rejected by single-field middleware.

Flow diagram for updated /group/participant middleware validation

flowchart LR
    A[Client POST /group/participant] --> B[ValidateMultipleNumbers participants]
    B -->|valid participants array| C[UpdateParticipant]
    B -->|invalid or empty participants| D[400 Bad Request]

    %% highlight consistency with create route
    E[Client POST /group/create] --> F[ValidateMultipleNumbers participants]
    F -->|valid| G[CreateGroup]
    F -->|invalid| D

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File-Level Changes

Change	Details	Files
Use array-aware JID validation middleware for the /group/participant route so participant actions work with the participants array payload.	Replaced single-string JID validation middleware with array-aware validation for the participants field on the /group/participant POST route. Removed the unused number field from the JID validation call for the participant route. Aligned participant route validation behavior with the existing /group/create route, which already uses array-aware validation for participants.	pkg/routes/routes.go

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#97	Update the POST /group/participant route to use an array-aware validation middleware for the participants field (ValidateMultipleNumbers("participants")) instead of the single-string middleware (ValidateJIDFields("number", "participants")), thereby allowing participant actions to succeed.	✅

Possibly linked issues

• POST /group/participant always returns 400 "participants is required and cannot be empty" (wrong validation middleware on route) #97: PR changes /group/participant to use ValidateMultipleNumbers("participants"), exactly fixing the issue’s reported validation bug.
• POST /group/participant always returns 400 "participants is required and cannot be empty" (wrong validation middleware on route) #97: PR switches /group/participant to array-aware validation, directly fixing the erroneous participants-required 400 error.

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