Upstream fragility: floating-upstream breakage detector

[whisper67265]

Problem

The plugin relies on undocumented Weblate internals (FormatsConf.FORMATS AST shape, WEBLATE_FORMATS, and weblate.urls.real_patterns list mutability). A scheduled pin bump to the latest PyPI Weblate could merge silently even when upstream has already broken those assumptions, leaving breakage to be discovered only at runtime.

Acceptance Criteria

• A contract-latest job runs before the pin-bump job in weblate-pin-bump.yml, installing the newest modern calver Weblate from PyPI and running contract tests
• scripts/check-weblate-internal-contract.sh supports --latest mode and exits non-zero with labeled contract failures when any assumption breaks
• Contract tests are marked weblate_contract and excluded from default pytest runs so normal CI is not gated on floating upstream

Implementation Notes

• Three focused contract tests mirror the plugin's real integration points: AST parsing of formats/models.py, weblate_formats_with_plugin_formats(), and append-to-real_patterns via _boost_endpoint_route()
• Failure messages prefix the broken contract ([FormatsConf.FORMATS AST], [WEBLATE_FORMATS], [weblate.urls.real_patterns]) to speed triage when upstream changes
• --latest resolves the newest modern calver release (year ≥ 2020) from PyPI JSON before installing and testing

References

• .github/workflows/weblate-pin-bump.yml
• .github/WORKFLOWS.md
• scripts/check-weblate-internal-contract.sh
• tests/test_weblate_internal_contract.py
• pyproject.toml