Add unified CXR runner and fix observation window

[joshuxchn]

Changes made prior to paper submission

• Added end-to-end experiment script for MIMIC-IV + MIMIC-CXR mortality prediction
  supporting 6 models with configurable observation windows. Extension of Rian's unified script
• Fixed observation window to skip admissions that start at or after effective_end to prevent a Polars Overflow error (negative integer window)

Tests:

• Ran BT, EHRMamba, JambaEHR with 24hr and 48hr windows, results in paper