feat: Add support for triggering error for PostgREST methods.

[dshukertjr]

What kind of change does this PR introduce?

Adds support to emulate errors.

What is the current behavior?

There are no way to emulate errors.

What is the new behavior?

You can emulate PostgrestExceptions with the postgrestExceptionTrigger parameter.

mockHttpClient = MockSupabaseHttpClient(
  postgrestExceptionTrigger: (schema, table, data, type) {
    // Simulate unique constraint violation on email
    if (table == 'users' && type == RequestType.insert) {
      throw PostgrestException(
        message: 'duplicate key value violates unique constraint "users_email_key"',
        code: '23505', // Postgres unique violation code
      );
    }
    
    // Simulate permission error for certain operations
    if (table == 'private_data' && type == RequestType.select) {
      throw PostgrestException(
        message: 'permission denied for table private_data',
        code: '42501', // Postgres permission denied code
      );
    }
  },
);