[DRAFT] Send existing conformance tests as context when rendering new conformance tests

[pedjaradenkovic]

Summary

First iteration of the conformance-testing improvement plan: when rendering conformance tests for a new functionality (FRID), the client now collects the module's existing conformance-test files and sends them to the API as context, so the generated tests don't duplicate coverage that already exists and follow the conventions of previous suites.

Pairs with Codeplain-ai/plain2code_rest_api#122 — the API field is optional, so this client change requires the backend to be deployed first, while the backend change is backward compatible with older clients.

Changes

• New ConformanceTests.fetch_all_existing_conformance_test_files(module_name): collects all text files across the module's per-FRID conformance test subfolders, keyed as <frid_subfolder>/<relative path>; excludes hidden folders (required-module copies) and conformance_tests.json.
• RenderConformanceTests._render_conformance_tests fetches these files (excluding the current FRID's own subfolder, so re-renders don't dedup against the tests being replaced), prints them, and passes them to the API.
• codeplain_REST_api.render_conformance_tests sends them as the new existing_conformance_tests_files payload field.

Execution model is intentionally unchanged: per-FRID folders, per-FRID test runs, regression walk, and fix loop all stay as-is (see CONFORMANCE_TESTING_IMPROVEMENT_PLAN.md for the roadmap).

Testing

• 5 new unit tests for the fetch helper; full suite 298 passed; black/isort/flake8/mypy clean.
• E2E against a locally running backend from the paired branch: rendered a fresh 2-FRID project; FRID 2's request contained FRID 1's test files, and its plan produced only tests for the new behavior (no duplicated happy-path test); render exited 0 with all conformance runs passing.