Support FromMethod on WriteAggregate for custom ID resolution

[jeremydmiller]

Summary

• [WriteAggregate] now supports FromMethod, FromRoute, FromHeader, and FromClaim for custom aggregate ID resolution, matching the existing behavior of [ReadAggregate]
• The fix is minimal: when a specific ValueSource has been set (e.g. via FromMethod), the inherited tryFindIdentityVariable() is used first, which respects the ValueSource property. Falls back to the existing FindIdentity() logic for full backward compatibility.

Example

public static class WriteAggregateFromMethodEndpoint
{
    public static Guid ResolveOrderId(ClaimsPrincipal user)
    {
        var claim = user.FindFirstValue("order-id");
        return claim != null ? Guid.Parse(claim) : Guid.Empty;
    }

    [WolverinePost("/orders/confirm-from-method"), EmptyResponse]
    public static OrderConfirmed Post(
        ConfirmOrderFromMethod command,
        [WriteAggregate(FromMethod = nameof(ResolveOrderId))] Order order)
    {
        return new OrderConfirmed();
    }
}

Closes #2451

Test plan

• All 619 Wolverine.Http.Tests pass (0 failures)
• 3 new integration tests: write with FromMethod, read with FromMethod, 404 on missing aggregate
• Verified backward compatibility — existing WriteAggregate behavior unchanged

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