fix: handle json.Marshal error in GetIsbSvcEnvVars

[syayi]

Summary

• GetIsbSvcEnvVars was silently ignoring the error from json.Marshal, causing encodedISBSvcConfig to be a base64-encoded "null" on failure — pods would start with a corrupted ISB config env var with no indication of the problem
• Replaced the inline json.Marshal call with the existing MustJSON helper, which is consistent with the MustHash/MustUnJSON panic-on-error pattern used elsewhere in this package
• Also removes the now-unused encoding/json import

Test plan

• Existing unit tests in isbs_config_test.go pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.15%. Comparing base (ab8e64c) to head (7429051).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3316      +/-   ##
==========================================
- Coverage   81.16%   81.15%   -0.01%     
==========================================
  Files         316      316              
  Lines       72828    72827       -1     
==========================================
- Hits        59108    59104       -4     
- Misses      13163    13164       +1     
- Partials      557      559       +2     

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