Implements the previously-`unimplemented!()`
`TestPlatform::prompt_for_paths` so tests can drive the platform Open
dialog deterministically.

Adds `TestAppContext::simulate_path_prompt_response` and
`did_prompt_for_paths`, mirroring the existing `prompt_for_new_path`
test helpers (`simulate_new_path_selection`). The simulated response
validates that callers don't return multiple paths when
`PathPromptOptions::multiple` is false.

Release Notes:

- N/A