Fix configurable default field errors for configurable variables

[iamvirul]

Purpose

• Compiler treated configurable module vars as mutable storage when referenced in record/object field default values, causing invalid access of mutable storage in the default value of a record field.
• Fixes [Bug]: Configurable Variables with Union Type Recognized as a Mutable Storage #44284 by ensuring configurable globals can safely back default initializers without forcing readonly.

Approach

• Updated IsolationAnalyzer to skip mutable-storage diagnostics when a record/object field default references a symbol flagged with Flags.CONFIGURABLE.
• Added regression coverage via test-src/statements/variabledef/configurable_record_field_default.bal and a new GlobalVarTest.testConfigurableVarInRecordFieldDefaultValue, guarding the reported scenario.

Samples

• tests/jballerina-unit-test/src/test/resources/test-src/statements/variabledef/configurable_record_field_default.bal demonstrates assigning a configurable string|string[]? to a record field default.

Remarks

• Verified with ./gradlew :jballerina-unit-test:test --tests org.ballerinalang.test.types.globalvar.GlobalVarTest on JDK 21.

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support ([Bug]: Configurable Variables with Union Type Recognized as a Mutable Storage #44284)
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.05%. Comparing base (3485c88) to head (a065e17).
⚠️ Report is 24 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #44403      +/-   ##
============================================
+ Coverage     75.02%   75.05%   +0.02%     
- Complexity    58602    58618      +16     
============================================
  Files          3600     3600              
  Lines        226681   226684       +3     
  Branches      29498    29500       +2     
============================================
+ Hits         170071   170131      +60     
+ Misses        47140    47078      -62     
- Partials       9470     9475       +5     

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[MaryamZi]

I'm not sure if this is the correct fix.

A configurable variable is implicitly final and immutable, so the check at L1315 should be sufficient. The issue here seems to be that the relevant flags aren't set when this check happens.

For example, the following works

configurable string|string[]? fooConfig = ();

isolated int[] x = [1, 2, 3];

function fn() returns string|string[]?  {
    lock {
        foreach var i in x {

        }
        
        // This is allowed because the flags are set by the time this check happens
        return fooConfig; 
    }
}