Feature: Modify the key of scheduling_map to std::string to avoid accessing SeparateScheduling as nullptr during startup

[yujun411522]

…essing SeparateScheduling as nullptr during startup

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.65000%. Comparing base (ce53eec) to head (7845ac4).

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #118         +/-   ##
===================================================
- Coverage   81.75622%   81.65000%   -0.10622%     
===================================================
  Files            855         855                 
  Lines          33891       33891                 
===================================================
- Hits           27708       27672         -36     
- Misses          6183        6219         +36     

Files	Coverage Δ
...untime/threadmodel/separate/separate_scheduling.cc	100.00000% <100.00000%> (ø)
...runtime/threadmodel/separate/separate_scheduling.h	33.33333% <ø> (ø)

... and 10 files with indirect coverage changes