simple_happy_path_client.py depends on removed mock_payment_handler

[ghunt-afi]

Description
It looks like mock_payment_handler was removed from
samples/rest/python/server/routes/discovery_profile.json.

This breaks the default test client example, since
samples/rest/python/client/flower_shop/simple_happy_path_client.py expects that key to be present.

Impact
The happy path client fails because it depends on mock_payment_handler being included in supported_handlers.

Workaround
For testing, updating

target_handler = supported_handlers[0]["id"]

works as expected.

Flagging this in case the happy path example needs to be updated or the handler restored.