fix(isJWT): validate header and payload decode to valid JSON objects

[abhu85]

Summary

• Fixes isJWT to properly validate that JWT header and payload decode to valid JSON objects
• Adds test cases for previously incorrect validations (e.g., foo.bar.baz, .babelrc.cjs)
• No breaking changes - only rejects strings that were incorrectly accepted before

Problem

The current isJWT validator only checks if the three dot-separated parts are valid Base64, but doesn't verify that the decoded header and payload are valid JSON objects as required by RFC 7519.

This causes false positives:

isJWT('foo.bar.')  // true (incorrect - "foo" is not valid JSON)
isJWT('.babelrc.cjs')  // true (incorrect)
isJWT('..')  // true (incorrect)

Solution

Per RFC 7519, a JWT consists of:

1. Header - MUST be a JSON object (contains alg, typ, etc.)
2. Payload - MUST be a JSON object (the claims)
3. Signature - Binary data (not required to be JSON)

The fix adds JSON parsing validation for the header and payload parts after Base64 decoding, ensuring they are valid JSON objects (not arrays, strings, or other types).

Test Plan

• All existing tests pass (317 passing)
• Added test cases for issue examples (foo.bar.baz, .babelrc.cjs, .., .t.)
• Linting passes
• Coverage maintained at 99%+

Compatibility

• No breaking changes for valid JWTs
• Backwards compatible - only rejects previously invalid inputs
• Works in both Node.js and browser environments

Fixes #2511

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (88e0d3d) to head (1602dfb).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2689   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2590      2612   +22     
  Branches       659       664    +5     
=========================================
+ Hits          2590      2612   +22     

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