Scheduler — Replace underscore-prefixed: Long tail (popup, helpers, header, utils)

[aleksei-semikozov]

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[Copilot]

Pull request overview

This PR continues the Scheduler internal refactor to eliminate underscore-prefixed members by renaming them to non-underscore equivalents and (where applicable) marking them private, while updating internal call sites and a legacy scheduler test accordingly.

Changes:

• Updated legacy Scheduler appointment popup test to use tryLockSaveChanges() instead of _tryLockSaveChanges().
• Refactored multiple Scheduler internal modules (appointment popup, compact helper, header/calendar, table creator, appointment data source) to rename underscore-prefixed helpers/fields and adjust call sites.
• Tightened encapsulation in several places by switching renamed members to private.

Reviewed changes

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Show a summary per file

File	Description
packages/devextreme/testing/tests/DevExpress.ui.widgets.scheduler/legacyAppointmentPopup.tests.js	Updates the legacy test to match the renamed appointment popup lock method.
packages/devextreme/js/__internal/scheduler/view_model/m_appointment_data_source.ts	Renames internal store-key/helper initialization methods and updates internal usage.
packages/devextreme/js/__internal/scheduler/m_table_creator.ts	Renames grouped-row creation helpers and marks them private, updating internal callers.
packages/devextreme/js/__internal/scheduler/m_compact_appointments_helper.ts	Renames internal template/button/tooltip helper methods and marks them private.
packages/devextreme/js/__internal/scheduler/header/m_header.ts	Renames underscore-prefixed header fields to private members and updates references.
packages/devextreme/js/__internal/scheduler/header/m_calendar.ts	Renames underscore-prefixed overlay/calendar fields to private members and updates references.
packages/devextreme/js/__internal/scheduler/appointment_popup/m_popup.ts	Renames underscore-prefixed popup helpers/handlers to private methods and updates internal references.
packages/devextreme/js/__internal/scheduler/appointment_popup/m_legacy_popup.ts	Same refactor as the non-legacy popup: renames underscore-prefixed internals and updates internal usage.