Add unit tests for studio session management

New tests:
- test_new_session_has_short_id
- test_new_interactive_session
- test_branch_name_format
- test_session_save_and_load
- test_session_mark_complete
- test_session_mark_paused
- test_list_empty_sessions
- test_backwards_compatibility_no_session_type

Added tempfile as dev dependency for temp directory tests.

crates/studio/Cargo.toml

@@ -17,3 +17,6 @@ serde_json = { workspace = true }
 uuid = { workspace = true }
 chrono = { version = "0.4", features = ["serde"] }
 termimad = "0.31"
+
+[dev-dependencies]
+tempfile = "3"