`tests/benchmark/` does not mirror `promptum.session` source structure

[deyna256]

The tests/benchmark/ directory contains tests for Report (session/report.py) and Prompt (session/case.py), both of which belong to the promptum.session package. A tests/session/ directory already exists for this package.

This split means session-layer tests are spread across two unrelated directories with no structural reason for the separation. When working on session code, it is unclear which test directory is authoritative. The convention that test structure mirrors source structure is violated.