[border-agent] update "ConnectionMode" in state bitmap when stopped (#11462)

This commit updates how the "ConnectionMode" field is set in the
Border Agent State Bitmap, which is advertised as the value of the
`sb` TXT key. In particular, when the Border Agent service is stopped
and therefore not accepting any connections, the value of this field
is now set to `kConnectionModeDisabled` to indicate this.

This commit also updates and enhances `test_border_agent` to validate
the State Bitmap entry in the TXT data, covering cases where the
device role changes or ePSKc support is enabled/disabled.

tests/scripts/thread-cert/node.py

@@ -1913,6 +1913,11 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect_done()
 
+    def get_ba_state(self):
+        states = [r'Disabled', r'Inactive', r'Active']
+        self.send_command('ba state')
+        return self._expect_result(states)
+
     def get_ephemeral_key_state(self):
         cmd = 'ba ephemeralkey'
         states = [r'Disabled', r'Stopped', r'Started', r'Connected', r'Accepted']