bridge tests: fake serves workflow contents; cover invalid-workflow status

b7a7e94 made the bridge read workflow files, which the strict fake Gitea
refused (7 red). The fake now serves contents (404 when absent), and new
tests cover: error posted with no runs, valid files add nothing, no repost,
.gitea/workflows wins over .github/workflows, and each workflow_problem
shape. pyyaml declared in dev extras (the bridge imports it).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -31,7 +31,7 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-dev = ["pytest>=8.3", "pytest-asyncio>=0.24", "ruff>=0.7", "mypy>=1.13"]
+dev = ["pytest>=8.3", "pyyaml>=6.0", "pytest-asyncio>=0.24", "ruff>=0.7", "mypy>=1.13"]
 
 [tool.ruff]
 line-length = 100