Allow clearing a campaign reminder on update

[jamescarr]

What

Changes CampaignUpdateInput.Reminder from *CampaignReminderInput to *Nullable[CampaignReminderInput].

Why

campaignUpdate treats an omitted reminder as "leave the existing reminder alone", so with a plain pointer there is no way to express "remove the reminder from this campaign" — the field either carries a value or disappears from the request. Wrapping it in Nullable lets callers send an explicit null.

This matters for the Terraform provider, where removing the reminder block from a campaign resource has to actually clear the reminder rather than silently keep it.

Notes

Everything else this originally carried (the CampaignReminder object, CampaignReminderInput, and the reminder enums) shipped in v2026.9.7, so this PR is now just the nullability change.

The generator template in templates/inputObjects.tpl carries the same per-field override, alongside the existing CheckPackageVersionUpdateInput one, so the type survives the next schema regeneration.

Testing

Two new mocked tests: one sends a full reminder and asserts the response parses, the other sends NewNullOf[CampaignReminderInput]() and asserts the request body contains "reminder":null.

[jamescarr]

Usage examples

Create a campaign with a weekly reminder

message := "Please complete your campaign checks."
slack := "platform-eng" // API stores this as "#platform-eng"

campaign, err := client.CreateCampaign(opslevel.CampaignCreateInput{
    Name:    "Upgrade to Rails 7",
    OwnerId: teamID,
    Reminder: &opslevel.CampaignReminderInput{
        Channels:            []opslevel.CampaignReminderChannelEnum{
            opslevel.CampaignReminderChannelEnumSlack,
            opslevel.CampaignReminderChannelEnumEmail,
        },
        Frequency:           1,
        FrequencyUnit:       opslevel.CampaignReminderFrequencyUnitEnumWeek,
        DaysOfWeek:          []opslevel.DayOfWeekEnum{opslevel.DayOfWeekEnumMonday, opslevel.DayOfWeekEnumThursday},
        TimeOfDay:           "09:30",
        Timezone:            "America/Chicago",
        Message:             &message,
        DefaultSlackChannel: &slack,
    },
})

DaysOfWeek must be omitted for day / month cadences (the API only accepts it for week).

Clear an existing reminder on update

_, err := client.UpdateCampaign(opslevel.CampaignUpdateInput{
    Id:       campaign.Id,
    Reminder: opslevel.NewNullOf[opslevel.CampaignReminderInput](), // marshals to `null`
})

Read it back

c, _ := client.GetCampaign(campaign.Id)
if c.Reminder != nil {
    fmt.Println(c.Reminder.FrequencyUnit, c.Reminder.NextOccurrence)
}