[Bug][SubscriptionBilling] Contract lines of type G/L Account never post to the selected G/L account (backport to releases/28.x)

[Miljan Milosavljević (miljance)]

What & why

• This is a backport of PR for 29: to 28.x [Bug][SubscriptionBilling] Contract lines of type G/L Account never post to the selected G/L account #9905
• post G/L Account contract lines to their own account. Subscription Billing contract lines of type "G/L Account" always posted to the Customer/Vendor Subscription Contract Account from the General Posting Setup, ignoring the account actually selected on the contract line.
• Without deferrals, CustomerDeferralsMngmt/VendorDeferralsMngmt now skip the General Posting Setup account lookup for "G/L Account" lines so the invoice posts directly to the line's account.
• With deferrals, a new "G/L Account No." field (27) on Cust./Vend. Sub. Contract Deferral carries the account from the sales/purchase line, and is surfaced on the Customer/Vendor Contract Deferrals pages.
• ContractDeferralsRelease now posts to that account when releasing the deferral, falling back to the General Posting Setup account when it is blank, and only requires the General Posting Setup account to be filled when no contract-line account is available.
• Added TDD tests to CustomerDeferralsTest and VendorDeferralsTest covering both the immediate-posting and deferral-release paths, with a shared fixture in ContractTestLibrary.

Linked work

Fixes #9885

How I validated this

• I read the full diff and it contains only changes I intended.
• I built the affected app(s) locally with no new analyzer warnings.
• I ran the change in Business Central and confirmed it behaves as expected.
• I added or updated tests for the new behavior, or explained below why none are needed.

What I tested and the outcome

• This backport is a clean cherry-pick of the commit from [Bug][SubscriptionBilling] Contract lines of type G/L Account never post to the selected G/L account #9905 onto releases/28.x — the diff is identical to the original PR's diff, with no conflicts and no context drift.
• The change itself (including the new tests in CustomerDeferralsTest and VendorDeferralsTest) was built, run, and validated on main as part of the original PR: posted contract invoices for G/L Account contract lines with and without deferrals and confirmed the line's G/L account is used for posting and for deferral release.
• The backport itself was not built or test-run against a 28.x environment.

Risk & compatibility

• Intentional behavior change: contract lines of type "G/L Account" without
  deferrals now post to the G/L account selected on the line instead of the
  Cust./Vend. Subscription Contract Account from General Posting Setup. Anyone
  who relied on the old (incorrect) routing will see revenue/cost on different
  accounts going forward.
• Schema: purely additive — new field 27 "G/L Account No." on the
  Cust./Vend. Sub. Contract Deferral tables. No breaking changes, no upgrade
  code needed: the field is blank on pre-existing deferral rows, and release
  falls back to the General Posting Setup account, so existing deferrals
  release exactly as before.
• Relaxed validation: Contract Deferrals Release no longer requires the
  General Posting Setup contract account when the deferral carries its own
  account — setups with a blank GPS account now succeed where they previously
  errored (intended).
• Credit memos: reversal copies the deferral record, so the new field is
  propagated automatically.
• Test library: ContractTestLibrary got a new overload; the existing
  CreateServiceObjectForGLAccountWithServiceCommitments signature is unchanged.
• No permissions, telemetry, or feature-flag impact. No follow-up work planned.
• The cherry-pick onto releases/28.x applied cleanly with no conflict resolution.

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