Support BackedEnum for discriminator values in STI

[roxblnfk]

🔍 What was changed

Added an ability to use BackedEnum as an STI discriminator.

📝 Checklist

• Closes #
• How was this tested:
  • Tested manually
  • Unit tests added

📃 Documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.60%. Comparing base (abaaa51) to head (5041fd9).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #566   +/-   ##
=========================================
  Coverage     91.59%   91.60%           
- Complexity     2013     2015    +2     
=========================================
  Files           131      131           
  Lines          5223     5229    +6     
=========================================
+ Hits           4784     4790    +6     
  Misses          439      439           

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[Copilot]

Pull request overview

This PR adds BackedEnum support for Single Table Inheritance discriminator values, allowing enum cases to be used when resolving child classes and scalar-backed values to be persisted through typecasting.

Changes:

• Normalizes BackedEnum discriminator values in STI class resolution.
• Adds BackedEnum uncasting in Typecast so enum cases persist as scalar backing values.
• Adds cross-driver functional coverage for string-backed, int-backed, and entity-property discriminator scenarios.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Mapper/Traits/SingleTableTrait.php	Resolves STI child classes using enum backing values.
src/Parser/Typecast.php	Adds enum case to scalar conversion during uncast.
tests/ORM/Functional/Driver/Common/Inheritance/STI/EnumDiscriminatorTest.php	Adds string-backed enum discriminator tests.
tests/ORM/Functional/Driver/Common/Inheritance/STI/IntEnumDiscriminatorTest.php	Adds int-backed enum discriminator tests.
tests/ORM/Functional/Driver/Common/Inheritance/STI/EnumPropertyDiscriminatorTest.php	Adds tests for enum discriminator exposed as entity property.
tests/ORM/Functional/Driver/Common/Inheritance/Fixture/EmployeeType.php	Adds string-backed enum fixture.
tests/ORM/Functional/Driver/Common/Inheritance/Fixture/EmployeeKind.php	Adds int-backed enum fixture.
tests/ORM/Functional/Driver/Common/Inheritance/Fixture/WorkerWithKind.php	Adds base fixture with enum discriminator property.
tests/ORM/Functional/Driver/Common/Inheritance/Fixture/BossWithKind.php	Adds child fixture for enum-property STI tests.
tests/ORM/Functional/Driver/MySQL/Inheritance/STI/*Enum*Test.php	Wires common enum STI tests for MySQL.
tests/ORM/Functional/Driver/Postgres/Inheritance/STI/*Enum*Test.php	Wires common enum STI tests for Postgres.
tests/ORM/Functional/Driver/SQLite/Inheritance/STI/*Enum*Test.php	Wires common enum STI tests for SQLite.
tests/ORM/Functional/Driver/SQLServer/Inheritance/STI/*Enum*Test.php	Wires common enum STI tests for SQL Server.

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