fix(crons): Log debug message when monitor config interval unit is invalid

[chiliec]

Description

Sentry::Cron::MonitorConfig.from_interval silently returns nil when given an unknown unit (e.g. :minutes instead of :minute), so the check-in is sent without a monitor config and there's no hint why. Per the discussion in #2117, this now emits a debug log through the SDK logger (no raise) listing the valid units, and still returns nil.

• resolves: Log when capture_checkin has bad arguments #2456

Tests

Added to spec/sentry/cron/monitor_config_spec.rb:

• logs a debug message for an invalid unit (fails without the change, passes with it)
• still returns nil when the SDK is not initialized

Locally, on Ruby 3.3.6:

• bundle exec rspec spec/sentry/cron/: 42 examples, 0 failures
• BUNDLE_GEMFILE=Gemfile.rubocop bundle exec rubocop on the changed files: no offenses
• full sentry-ruby suite: the only failures are the Redis specs (no Redis server on my machine) and one GraphQL spec that also fails on master