Refactor Omnichannel modal validation & internal logic for clarity

Description:

This issue proposes a small internal refactor in multiple Omnichannel modal components to improve validation logic, readability, and maintainability.
The changes do not affect UI/UX or user-facing behavior and are limited to internal logic improvements (mainly form validation and side-effect handling).

The refactor removes reliance on potentially stale react-hook-form error state inside submit handlers and avoids confusing dependency patterns in useEffect.

Affected files:

• CloseChatModal.tsx

• ForwardChatModal.tsx

• ReturnChatQueueModal.tsx

• EnterpriseDepartmentsModal.tsx

Steps to reproduce:

• Open any Omnichannel modal (Close Chat / Forward Chat / Return to Queue).

• Submit the form with missing required fields (comment, tags, or subject when transcript email is enabled).

• Observe how validation is handled internally.

Expected behavior:

• Validation should be evaluated synchronously inside submit handlers.

• Submission should proceed only when all validation conditions pass.

• Side-effects (like setting default email subject) should not unintentionally override user input.

• Internal logic should be easy to understand and maintain for future contributors.

Actual behavior:

• Validation logic relied on errors state immediately after calling setError, which can reference stale state.

• Conditional checks were inverted or incomplete in some cases.

• useEffect dependency on subject could cause unexpected resets of the subject field.

• While the UI works, the internal logic is harder to reason about and more error-prone.

• No UI regression observed. No screenshots required.

Server Setup Information:

• Version of Rocket.Chat Server: N/A (code-level refactor)

• License Type: N/A

• Number of Users: N/A

• Operating System: N/A

• Deployment Method: N/A

• Number of Running Instances: N/A

• DB Replicaset Oplog: N/A

• NodeJS Version: N/A

• MongoDB Version: N/A

• Client Setup Information

• Desktop App or Browser Version: N/A

• Operating System: N/A

Additional context

• changes are non-breaking and do not modify UI or business behavior.

• The refactor improves:

• readability

• contributor friendliness

• correctness of validation flow

A pull request has already been prepared following this issue for easier review and discussion.

Closes #38432

Relevant logs:

Not applicable.
This is a code refactor / logic cleanup with no runtime errors or logs involved.

Related PR: https://github.com/RocketChat/Rocket.Chat/pull/38387

Start by reviewing CloseChatModal.tsx, ForwardChatModal.tsx, ReturnChatQueueModal.tsx, and EnterpriseDepartmentsModal.tsx, then compare the prepared changes in PR #38387. Trace each submit handler and the subject-related useEffect; the work is done when validation is synchronous, user input is preserved, and the existing behavior remains unchanged.