Fix overflow in AccessFailedAsync when DefaultLockoutTimeSpan is large

[GOVINSAGA]

Summary

Fixes #60181.

UserManager.AccessFailedAsync sets the lockout end date with:

UtcNow().Add(Options.Lockout.DefaultLockoutTimeSpan)

When DefaultLockoutTimeSpan is large enough that the result exceeds DateTimeOffset.MaxValue, this throws ArgumentOutOfRangeException. A legitimate scenario is setting DefaultLockoutTimeSpan = TimeSpan.MaxValue to lock an account out indefinitely (e.g. after repeated failed MFA attempts). Today that crashes instead of locking the user out.

Change

Clamp the computed lockout end date to DateTimeOffset.MaxValue when the configured timespan would overflow:

var now = UtcNow();
var lockoutEndDate = Options.Lockout.DefaultLockoutTimeSpan < DateTimeOffset.MaxValue - now
    ? now.Add(Options.Lockout.DefaultLockoutTimeSpan)
    : DateTimeOffset.MaxValue;

The overflow check computes headroom as DateTimeOffset.MaxValue - now (always a valid TimeSpan), so the check itself cannot overflow.

Tests

Adds LockoutWithMaxTimeSpanDoesNotOverflow to the shared UserManagerSpecificationTestBase suite, so every store implementation (in-memory, EF Core) is covered. It sets DefaultLockoutTimeSpan = TimeSpan.MaxValue, triggers a lockout via AccessFailedAsync, and asserts the call succeeds and the lockout end date is DateTimeOffset.MaxValue.

[dotnet-policy-service]

Thanks for your PR, @GOVINSAGA. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.