[18.0][FIX] account_invoice_custom_rounding: keep tax base-line contract

[grojof]

Summary

• Harden tax base line preparation in account_invoice_custom_rounding.
• Handle missing transient records safely while preserving expected tax computation keys.
• Add focused tests for missing-record fallback behavior.

Why

Avoid fragile behavior in rounding-related tax computation paths and keep contract stable.

Changes

• account_invoice_custom_rounding/models/account_tax.py
• account_invoice_custom_rounding/tests/test_account_invoice_custom_rounding.py

Merge order

3/4 (after PR 1 and PR 2)

Depends on:

• [18.0][FIX] account_invoice_auto_send_by_email: set trusted outgoing bank in tests #2271
• [18.0][FIX] tests: stabilize account_billing/custom_rounding/show_currency_rate #2272

[grojof]

Cross-reference: migration follow-up has been posted on #2063 for rerun with the updated 18.0 context (after #2271-#2274 stabilization).

Ref: #2063 (comment)