[Subscription Billing] Fix deferral creation, release, and credit memo handling

[miljance]

Summary

This pull request introduces a new data upgrade step to populate posting group fields in contract deferral records, along with some code clean-up and improvements/fixes across related modules.

1. Extensibility: Five core deferral objects need integration points (events) for ISV extensions to customize behavior during deferral creation and release processes.

2. Missing Posting Groups in Deferrals: Contract deferrals cannot be released when the original posted documents are unavailable (e.g., in data migration scenarios), because posting group information is retrieved from posted invoice/credit memo lines instead of being stored in the deferral entries.

3. Zero-Amount Deferrals: The system creates unnecessary deferral entries and G/L postings for contract lines with 0.00 amounts, leading to excessive ledger entries with no financial impact.

4. Inconsistent Partial Period Handling: Vendor contract deferrals handle partial billing periods inconsistently—some invoices are deferred based on full months while others are calculated daily, causing incorrect revenue recognition.

5. Credit Memo Deferral Gap: Credit memos with amounts exceeding the original invoice amount do not generate corresponding deferral entries, resulting in incomplete revenue recognition.

6. Double Deferral Generation: When both contract-based deferrals and standard deferral codes are used, deferrals are incorrectly created in both systems simultaneously.

Work Item(s)

Fixes #6281
AB#623188

[AndersLarsenMicrosoft]

From the log

AA0215 The file DeferralPostingPreviewBinding.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewBinding.Codeunit.al.
AA0215 The file DeferralPostingPreviewHandler.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewHandler.Codeunit.al.
AA0215 The file DeferralPostingPreviewSubscr.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewSubscr.Codeunit.al.

[miljance]

From the log

AA0215 The file DeferralPostingPreviewBinding.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewBinding.Codeunit.al. AA0215 The file DeferralPostingPreviewHandler.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewHandler.Codeunit.al. AA0215 The file DeferralPostingPreviewSubscr.Codeunit.al has an incorrect name. The valid name is DeferralPostPreviewSubscr.Codeunit.al.

Fixed