This PR fixes a regression in System.Text.Json 9 where flags enums are serialized differently than in previous versions when using JsonStringEnumConverter. The issue occurs specifically when there are combinations of bits where not all individual bits have their own explicit enum value.

In .NET 8, a flags enum value would be serialized as a comma-separated list of the named values that make up the combination. In .NET 9, if there's a combination where some bits don't have explicit individual values, the serialization would fall back to the numeric representation.

For example:

[Flags]
public enum MyEnum
{
    UNKNOWN = 0,
    BIT0    = 1,
    // No explicit value for bit 1
    BIT2    = 4,
    BIT3    = 8,
    BITS01  = 3,  // Combination of BIT0 and bit 1
}

var value = MyEnum.BITS01 | MyEnum.BIT3;

// .NET 8 output: "BITS01, BIT3"
// .NET 9 output (before this fix): "11" (numeric value)

Root Cause

The regression was introduced in PR #105032 which replaced the usage of Enum.Parse with a bespoke implementation to support the new JsonStringEnumMemberNameAttribute feature.

The issue is in the IsDefinedValueOrCombinationOfValues method which determines if a value should be serialized as a string or number. The new implementation was too strict, requiring that every bit in the value could be matched to an exact field or combination of fields.

Fix

The fix modifies the IsDefinedValueOrCombinationOfValues method to be more lenient when checking flags enum values:

1. First, try to match the value to an exact field for performance
2. Then, attempt bit-by-bit matching as before
3. If there are remaining unmatched bits, use a more permissive approach that accepts values as long as at least one field can partially match the value

This approach ensures we maintain the same behavior as in .NET 8 where any flags enum with at least one named field would be formatted as a string rather than falling back to the numeric representation.

Testing

Added test cases for:

• Basic flag enum combinations with all bits defined
• Flag enum with missing bit definitions
• Complex flags enum with various combination fields

Fixes #114770.

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