`session/case.py` module name does not reflect its exported class `Prompt`

[deyna256]

The module src/promptum/session/case.py exports a single class Prompt. The filename case.py is a legacy name from when the class was called TestCase. After the rename to Prompt, the module was not renamed, creating a mismatch: the import path promptum.session.case gives no indication that it contains Prompt. This makes the codebase harder to navigate and the module harder to locate by name.