Fix parsing map keys in fromBalString method

[Nadeeshan96]

Purpose

Describe the problems, issues, or needs driving this feature/fix and include links to related issues.

$title
Fixes #41700

Approach

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Samples

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Remarks

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Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (04c07e4) 76.70% compared to head (a0c6852) 76.73%.
Report is 32 commits behind head on master.

❗ Current head a0c6852 differs from pull request most recent head 985e312. Consider uploading reports for the commit 985e312 to get more accurate results

Files	Patch %	Lines
.../io/ballerina/runtime/internal/BalStringUtils.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #41799      +/-   ##
============================================
+ Coverage     76.70%   76.73%   +0.03%     
+ Complexity    52736    52733       -3     
============================================
  Files          2878     2878              
  Lines        198759   198742      -17     
  Branches      25841    25834       -7     
============================================
+ Hits         152464   152513      +49     
+ Misses        37874    37803      -71     
- Partials       8421     8426       +5     

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[warunalakshitha]

LGTM