fix: support dynamic function callbacks in equal/notEqual validators

[GhagSagar23]

Connection with issue(s)

Close #63

Solution description

This PR updates the EqualValidator and NotEqualValidator classes to support dynamic comparison targets.

When you pass an argument to these validators (e.g., FormBuilderValidators.equal(password)), it is evaluated once during the parent widget's build tree execution. If the target field changes later (such as a password field during registration), the confirmation field's validator remains stuck on the stale initial value unless a full state rebuild is triggered.

Changes:

1. Modified EqualValidator and NotEqualValidator to inspect if the comparison value is a Function. If it is, the validator evaluates it dynamically at validation runtime (using (value as Function)()) instead of statically.
2. Extracted the resolution check to a private getter _resolvedValue and refactored validation methods using expression bodies.
3. Added type annotations to satisfy the strict static analyzer rules of the package.
4. Added new test suites in test/src/core/equal_validator_test.dart and test/src/core/not_equal_validator_test.dart confirming dynamic callbacks validate properly when comparison targets are updated.

Screenshots or Videos

(Not applicable — pure logic / functional package change)

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

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Test Verification Output

All 984 unit tests passed successfully under static analyzer rules (fvm flutter analyze and fvm flutter test).

Click to expand unit test run logs

Analyzing form_builder_validators-fix-63...                     
No issues found! (ran in 2.2s)

00:01 +121: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return null if the value is equal to the specified value
00:01 +122: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return error if the value is not equal to the specified value
00:01 +123: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return null if the numeric value is equal to the specified value
00:01 +124: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return error if the numeric value is not equal to the specified value
00:01 +125: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return error if the value is null and null check is enabled
00:01 +126: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return null if the value is null and null check is disabled
00:01 +127: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return null if the value is empty and null check is disabled
00:01 +128: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return error if the value is empty and null check is enabled
00:01 +129: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should return custom error message if the value is not equal to the specified value
00:01 +130: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/equal_validator_test.dart: EqualValidator - should support dynamic function callbacks for comparison value
00:02 +162: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return null if the value is not equal to the specified value
00:02 +165: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return null if the value is null and null check is disabled
00:02 +166: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return null if the value is null and null check is disabled
00:02 +169: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return null if the integer value is not equal to the specified value
00:02 +171: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return errorText if the integer value is equal to the specified value
00:02 +172: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return errorText if the integer value is equal to the specified value
00:02 +175: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return errorText if the double value is equal to the specified value
00:02 +177: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return null if the list value is not equal to the specified value
00:02 +179: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return errorText if the list value is equal to the specified value
00:02 +180: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should return errorText if the list value is equal to the specified value
00:02 +181: /Users/ghag23/Community/form_builder_validators-fix-63/test/src/core/not_equal_validator_test.dart: NotEqualValidator - should support dynamic function callbacks for comparison value
...
00:13 +984: All tests passed!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.21%. Comparing base (986934a) to head (99f7077).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #163   +/-   ##
=======================================
  Coverage   99.20%   99.21%           
=======================================
  Files          95       95           
  Lines        1012     1018    +6     
=======================================
+ Hits         1004     1010    +6     
  Misses          8        8           

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