Add campaign reminder configuration support

[jamescarr]

Summary

• Adds CampaignReminderInput and wires a reminder field into CampaignCreateInput and CampaignUpdateInput, enabling recurring campaign reminders (Slack, email, Microsoft Teams) via the existing campaignCreate / campaignUpdate mutations.
• CampaignUpdateInput.Reminder uses *Nullable[CampaignReminderInput] so a reminder can be explicitly cleared.
• Changes Campaign.Reminder to *CampaignReminder so "no reminder" is distinguishable from a zero-value reminder, and adds the read-side DefaultMicrosoftTeamsChannel field.

Notes

• daysOfWeek is only valid for a weekly cadence; callers should omit it otherwise (the API rejects it for daily/monthly). This matches the validation enforced in the terraform-provider-opslevel consumer.
• The API normalizes defaultSlackChannel by prepending #.

Test plan

• go build ./...
• go vet ./...
• go test ./... (added TestCreateCampaignWithReminder and TestUpdateCampaignClearReminder; updated mock templates for the new defaultMicrosoftTeamsChannel selection)
• gofumpt clean

Made with Cursor

[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)
}