fix(accounting)!: correct RECEIPT contract and document namespaced TaxNumberType

[ryanduguid]

Summary

Correct the Accounting API contract for receive-money linked transactions by adding RECEIPT to LinkedTransaction.SourceTransactionTypeCode. Document the observed TAXNUMBERTYPE/SSN form in the Contact.TaxNumberType description without changing that enum.

The branch is refreshed against upstream master at 448060d (19.0.0). The diff against that base remains confined to these accounting-spec changes.

Compatibility

The API diff correctly identifies four response-enum expansion findings for RECEIPT. Clients generated from the previous closed enum may reject that value, which is the failure reported in xero-python#206. The contribution explicitly declares this with a BREAKING CHANGE commit footer and a conventional breaking-change title. Under this repository's documented release rules, that requests major-version treatment; maintainers must review the release impact. No validation rule has been removed or weakened.

Related reports: xero-python#203 and xero-python#205. Companion runtime workaround: XeroAPI/xero-python#243.

Verification

• bash scripts/api-diff/api-diff.test.sh: all 9 tests pass, including the explicit fail-on-breaking override.
• git diff --check: passes.
• Current diff against upstream reviewed: only xero_accounting.yaml changes.
• Docker/oasdiff and the private code-generation service are unavailable locally; GitHub Actions provides those checks.

Maintainer review

Confirm that RECEIPT and the observed namespaced tax-number form match the supported Accounting API contract, and confirm the requested compatibility/version treatment before merging.

[ryanduguid]

The pull-request workflow runs are currently waiting for maintainer approval. Please approve them when convenient so CI can run against this revision.

[ryanduguid]

Refreshed this PR against current master (19.0.0) at a554e23. The net diff remains the RECEIPT enum correction and TaxNumberType description.

I checked the failed API diff: all four errors are response-enum expansion findings for RECEIPT. The contribution now explicitly declares that compatibility impact using the documented BREAKING CHANGE convention and a matching PR title. Maintainers still need to confirm the major-version treatment; no validation rule has been weakened. All 9 conventional-commit logic tests pass locally.

The new OpenAPI Spec Diff Check and PR Linting runs are awaiting maintainer approval (33996696219 and 33996696243). Please approve those runs when convenient so this revision can be verified. Codegen Repo Validation is running separately.