[ffigen] Generate a non-throwing fromValueOrNull on Dart enums #3712

Problem

For each C enum, ffigen generates a Dart enum with a fromValue constructor that throws on any value it doesn't recognise:

static ATEntryPoint fromValue(int value) => switch (value) {
  0 => AutotestMenu,
  1 => NoSearch,
  // ...
  _ => throw ArgumentError('Unknown value for ATEntryPoint: $value'),
};

That's the right default when the native side only ever produces valid members. But many enum values come from outside the current build, for example:

• records saved by a different version of the native library, with ids the current enum doesn't name
• values read from files, labels or the network and cast to the enum without being checked
• enums that gain new members over time, where older bindings should handle the new ids gracefully

In these cases an unknown value is expected, not a bug. The caller wants to check for it, keep the raw int, and carry on, not catch an ArgumentError.

Proposal

Generate a second static method next to fromValue that returns null instead of throwing:

static ATEntryPoint? fromValueOrNull(int value) => switch (value) {
  0 => AutotestMenu,
  1 => NoSearch,
  // ...
  _ => null,
};

fromValue could then just call it:

static ATEntryPoint fromValue(int value) =>
    fromValueOrNull(value) ??
    (throw ArgumentError('Unknown value for ATEntryPoint: $value'));

That keeps the generated output about the same size, since the switch appears only once.

Considerations

• Naming: fromValueOrNull follows int.tryParse / firstOrNull style; tryFromValue would work too.
• Opt-in vs. default: the method is small and has no effect unless called, so generating it always seems reasonable. If output size is a concern, it could sit behind an option.
• Enums with duplicate values: these should behave the same as fromValue does today (it returns the canonical member).
• Compatibility: the change only adds a method; existing fromValue behaviour stays the same.