The slack sub-tests in TestDiscordOnly_CreateRejectsNonDiscord and
TestBlocker3_CreateProviderRejectsNonDiscordWithSecurityEvents were
omitting the required token field from their request payloads.
CreateProvider enforces that Slack providers must have a non-empty
token (the webhook URL) at creation time. Without it the service
returns "slack webhook URL is required", which the handler does not
classify as a 400 validation error, so it falls through to 500.
Add a token field to each test struct, populate it for the slack
case with a valid-format Slack webhook URL, and use
WithSlackURLValidator to bypass the real format check in unit tests —
matching the pattern used in all existing service-level Slack tests.
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