Fix: Resolve compilation type errors in MessageContext.tsx

[MohitBareja16]

Problem

The MessageContext.tsx file contains TypeScript inconsistencies that cause active compilation errors:

1. optimisticMessage.senderId strictly expects a string, but currentUser.id is typed as string | undefined.
2. The MessageContextType interface defines markAllAsSeen as returning a Promise<any>, but its actual implementation in the component returns void.

These mismatches prevent the application from successfully building and fail the strict typing guarantees.

Solution

1. Add an empty string fallback (currentUser.id || "") when instantiating the optimistic message to guarantee a string assignment.
2. Update the MessageContextType interface so that markAllAsSeen explicitly returns void, correctly aligning the contract with the implementation.