[DON-1368] move CalendarDayCellTestTag

[peterliu (peterInTown)]

This pull request refactors the CalendarDayCellTestTag enum by moving it from an internal scope to a dedicated file and updates its usage accordingly. This change improves code organization and reusability.

Code Organization:

• Created a new file, CalendarDayCellTestTag.kt, and moved the CalendarDayCellTestTag enum to this file to make it more modular and accessible.
• Removed the inline definition of CalendarDayCellTestTag from BpkCalendarDayCell.kt and replaced it with an import statement referencing the new file. [1] [2]

• Check this if you have read and followed the contributing guidelines

Remember to include the following changes:

• Component README.md
• Tests

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[skyscanner-backpack-bot]

	Fails
🚫	These new files do not include the license heading: backpack-compose/src/main/kotlin/net/skyscanner/backpack/compose/calendar/CalendarDayCellTestTag.kt

	Warnings
⚠️	One or more component files were updated, but the tests weren't updated. If your change is not covered by existing tests please add snapshot tests.
⚠️	One or more component files were updated, but the docs screenshots weren't updated. If the changes are visual or it is a new component please regenerate the screenshots via ./gradlew recordScreenshots.
⚠️	One or more component files were updated, but README.md wasn't updated. If your change contains API changes/additions or a new component please update the relevant component README.
⚠️	One or more package files were created, but BpkComposeComponentUsageDetector.kt wasn't updated. If your component is an equivalent of a core component please add it to the detector.

Generated by 🚫 Danger Kotlin against 2df43f2

[skyscanner-backpack-bot]

	Fails
🚫	These new files do not include the license heading: backpack-compose/src/main/kotlin/net/skyscanner/backpack/compose/calendar/BpkCalendarDayCellTestTag.kt

	Warnings
⚠️	One or more component files were updated, but the tests weren't updated. If your change is not covered by existing tests please add snapshot tests.
⚠️	One or more component files were updated, but the docs screenshots weren't updated. If the changes are visual or it is a new component please regenerate the screenshots via ./gradlew recordScreenshots.
⚠️	One or more component files were updated, but README.md wasn't updated. If your change contains API changes/additions or a new component please update the relevant component README.
⚠️	One or more package files were created, but BpkComposeComponentUsageDetector.kt wasn't updated. If your component is an equivalent of a core component please add it to the detector.

Generated by 🚫 Danger Kotlin against df50c36

[skyscanner-backpack-bot]

	Warnings
⚠️	One or more component files were updated, but the tests weren't updated. If your change is not covered by existing tests please add snapshot tests.
⚠️	One or more component files were updated, but the docs screenshots weren't updated. If the changes are visual or it is a new component please regenerate the screenshots via ./gradlew recordScreenshots.
⚠️	One or more component files were updated, but README.md wasn't updated. If your change contains API changes/additions or a new component please update the relevant component README.
⚠️	One or more package files were created, but BpkComposeComponentUsageDetector.kt wasn't updated. If your component is an equivalent of a core component please add it to the detector.

Generated by 🚫 Danger Kotlin against a5866d2

[skyscanner-backpack-bot]

	Warnings
⚠️	One or more component files were updated, but the tests weren't updated. If your change is not covered by existing tests please add snapshot tests.
⚠️	One or more component files were updated, but the docs screenshots weren't updated. If the changes are visual or it is a new component please regenerate the screenshots via ./gradlew recordScreenshots.
⚠️	One or more component files were updated, but README.md wasn't updated. If your change contains API changes/additions or a new component please update the relevant component README.
⚠️	One or more package files were created, but BpkComposeComponentUsageDetector.kt wasn't updated. If your component is an equivalent of a core component please add it to the detector.

Generated by 🚫 Danger Kotlin against 7ce314c