fix: record sessions to bare relative filenames

[Aditya Pratap Singh (akkupratap323)]

Why are these changes needed?

Recording a session to session.json raises ValueError because finalize() tries to create an empty parent directory. Create a directory only when the session path has a parent, so a bare filename saves into the working directory.

The regression records and replays a real interaction with the offline replay client, covering bare filenames, ./ paths, missing nested directories and absolute paths. The bare-filename case fails on the original source while the other three pass; all four pass with this change.

Related issue number

Fixes #8346

Checks

• No documentation change is needed: this restores normal filename behavior without changing the public API.
• Added regression coverage. Complete recorder test file: 6 passed, run through UV on Python 3.11.14 / Windows. Ruff 0.4.8 check and format pass for both changed files.
• All remote checks have passed (pending CI).

The full extension suite and live-provider integration tests were not run. #7797's encoding change is separate; this patch leaves file encoding unchanged. Investigated and implemented with Codex assistance; tests ran locally against the real classes.