types(chat): narrow IMessage.messageType using literal union types

[nitin-kumar-code]

This PR improves type safety in the chat feature by narrowing
IMessage.messageType to a literal union of the defined message types.

Changes:

• Marked MESSAGE_TYPE_* constants with as const
• Introduced a ChatMessageType union type
• Updated IMessage.messageType to use the union
• Aligned action typings to use the new type

Tested locally:

• Chat messages render correctly
• Sending local, remote and error messages works as before
• No type errors or lint warnings

No runtime behavior changes were introduced.

This prevents accidental use of invalid messageType values,
strengthens the chat data model, and improves maintainability.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[nitin-kumar-code]

Hi @saghul, @damencho
Could you please review and let me know if any changes are required?