fix: save button stays enabled after saving availability (#26708)

[ousamabenyounes]

What does this PR do?

• Fixes The save button on the availability schedule page remains enabled after successfully saving changes. #26708

The Save button on the availability schedule page remained enabled after saving changes due to a property name mismatch in the formHasChanges comparison.

Root cause

The comparison checked initialValuesRef.current.availability, but the form field is named schedule. Since availability was never populated from defaultValues, it was always undefined, making the comparison always true.

- JSON.stringify(initialValuesRef.current.availability)
+ JSON.stringify(initialValuesRef.current.schedule)

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Navigate to /availability, click any schedule
2. Make a change, click Save
3. Before: button stays enabled. After: button disables correctly.

Files changed

File	Change
packages/platform/atoms/availability/AvailabilitySettings.tsx	Fix .availability → .schedule
packages/platform/atoms/availability/AvailabilitySettings.test.tsx	7 unit tests for formHasChanges logic

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