Fix accessor signatures for string/octstr attributes.

[bzbarsky-apple]

See project-chip/connectedhomeip#28357: we are not mutating the actual value the caller will observe.

Note: This is expected to change Matter codegen. The previous codegen was broken.

[codecov-commenter]

Codecov Report

Merging #1122 (36247e4) into master (a75c922) will decrease coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

❗ Current head 36247e4 differs from pull request most recent head 7c7bd5d. Consider uploading reports for the commit 7c7bd5d to get more accurate results

@@            Coverage Diff             @@
##           master    #1122      +/-   ##
==========================================
- Coverage   65.29%   65.27%   -0.02%     
==========================================
  Files         170      170              
  Lines       18184    18189       +5     
  Branches     3915     3916       +1     
==========================================
  Hits        11873    11873              
- Misses       6311     6316       +5     

Files Changed	Coverage Δ
...r/app/zap-templates/common/attributes/Accessors.js	25.00% <0.00%> (-2.28%)	⬇️