Mirrors dotnet/runtime#124638. LOGGEN007 previously rejected all generic logging methods unconditionally. The restriction is narrowed to only block allows ref struct constraints, which can't be stored as fields in the generated state struct.

Changes

• Parser: Remove Arity > 0 → LOGGEN007. Populate LoggingMethod.TypeParameters from IMethodSymbol.TypeParameters, serializing all constraint kinds. Detect allows ref struct via reflection on ITypeParameterSymbol.AllowsRefLikeType (Roslyn 4.9+; no-ops on older Roslyn).
• Diagnostic: Rename LoggingMethodIsGeneric → LoggingMethodHasAllowsRefStructConstraint with updated message; update docs/list-of-diagnostics.md.
• Model: Add LoggingMethodTypeParameter (Name, Constraints); add TypeParameters list to LoggingMethod.
• Emitter: Emit <T1, T2, ...> and properly indented where T : ... clauses on the generated partial method signature. Use Out(char) for single-character output.
• Tests: MethodGeneric now expects success; add MethodGenericWithConstraints, MethodGenericMultipleTypeParams, MethodGenericWithAllowsRefStructConstraint (skips on Roslyn < 4.9), and GenericTestExtensions.cs for emitter validation.

Example

// Previously LOGGEN007 — now generates correct code
[LoggerMessage(0, LogLevel.Trace, "Received {Code} on {Channel}")]
public static partial void PacketReceived<TCode>(
    ILogger logger, TCode code, ConnectionChannel channel)
    where TCode : struct, Enum;

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