fix: don't compare strings with != in JSON codec

[anthony-swirldslabs]

Description:
Implementing correct string handling in JSON codec toJSON() method to avoid using != for string comparison.

Related issue(s):

Fixes #307

Notes for reviewer:
I manually inspected the generated JSON codecs for MessageWithString and Everything, and they look fine now.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

JUnit Test Report

   78 files  ±0     78 suites  ±0   3m 59s ⏱️ ±0s
1 347 tests ±0  1 343 ✅ ±0   4 💤 ±0  0 ❌ ±0 
7 224 runs  ±0  7 204 ✅ ±0  20 💤 ±0  0 ❌ ±0 

Results for commit ae81d31. ± Comparison against base commit 14bc7cd.

This pull request removes 2 and adds 1 tests. Note that renamed tests count towards both.

, 1
com.hedera.pbj.runtime.Utf8ToolsTest ‑ [4] 

com.hedera.pbj.runtime.Utf8ToolsTest ‑ [4] 
, 1

[github-actions]

Integration Test Report

    409 files  ±0      409 suites  ±0   16m 10s ⏱️ + 1m 50s
114 853 tests ±0  114 853 ✅ ±0  0 💤 ±0  0 ❌ ±0 
115 094 runs  ±0  115 094 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit ae81d31. ± Comparison against base commit 14bc7cd.

This pull request removes 3 and adds 3 tests. Note that renamed tests count towards both.

com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [1] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f5927bea6e0@5bb58db8
com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [2] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f5927bea910@7c41e50d
com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [3] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f5927beab40@5eeab201

com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [1] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f92b0bc4000@59621804
com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [2] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f92b0bc4230@589f936c
com.hedera.pbj.integration.test.ParserNeverWrapsTest ‑ [3] com.hedera.pbj.integration.test.ParserNeverWrapsTest$$Lambda/0x00007f92b0bc4460@3d657ebd