fix(monitors): Coerce numeric string interval count to integer

[Ritiky23]

Summary

Fixes an issue where updating a Cron monitor interval count in the UI or via form-encoded request pipelines results in a validation error: Invalid schedule for schedule unit count.

Context

When a user updates a monitor's schedule (e.g., from "Every 5 minutes" to "Every 6 minutes"), the UI/API request payload can sometimes send the interval count as a numeric string (e.g., "6" instead of 6).

Previously, ConfigValidator.validate had a strict type check:
if not isinstance(schedule[0], int): raise ValidationError
This rejected valid numeric strings that just needed type coercion.

Changes

• Updated ConfigValidator.validate in src/sentry/monitors/validators.py to check if schedule[0] is a string and attempt to coerce it to an integer.
• Non-numeric strings (e.g. "abc") still raise the appropriate ValidationError.
• Added unit tests in tests/sentry/monitors/test_validators.py to cover both success (numeric strings coerced to integers) and failure (non-numeric strings rejected) cases.

Closes #119214

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[bruno-garcia]

@sentry review
bug bot

[getsantry]

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