fix: prevent sending messages with attachments multiple times by adding current state check

[VityaSchel]

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Description

I have tested issue described in #2744 and figured out that the problem was in onSendMessage() function taking long time to complete (and thus reset composition box input) because of getFiles() async function which scales and stores attachments locally before sending.

I have added a simple check to CompositionBoxInner class that prevents user from sending multiple messages. I think the best solution would be to move all attachments logic to sending stage so that visually messages would be sent immediately, but I didn't want to change a lot of logic.

Related: #2744

[pottsandpans]

This will be tracked internally on Jira (SES-1939) for code review.