test: add headless_prompt stub to in-test AgentProvider subclasses

The new abstract headless_prompt method broke two test files that
define minimal _Provider subclasses of AgentProvider, causing
collection-time TypeErrors that aborted the whole unit suite.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WL77TgFxKbs3cidGMG9dz7

tests/unit/test_docker_provision_git_user.py

@@ -38,6 +38,7 @@ class _Provider(AgentProvider):
     def provision_prompt(self, plan, bottle): ...  # type: ignore[override]
     def provision(self, plan, bottle): ...  # type: ignore[override]
     def provision_supervise_mcp(self, plan, bottle, supervise_url): ...  # type: ignore[override]
+    def headless_prompt(self, prompt): return []  # type: ignore[override]
 
 
 _PROVIDER = _Provider()

tests/unit/test_smolmachines_provision.py

@@ -49,6 +49,7 @@ class _Provider(AgentProvider):
     def provision_prompt(self, plan, bottle): ...  # type: ignore[override]
     def provision(self, plan, bottle): ...  # type: ignore[override]
     def provision_supervise_mcp(self, plan, bottle, supervise_url): ...  # type: ignore[override]
+    def headless_prompt(self, prompt): return []  # type: ignore[override]
 
 
 _PROVIDER = _Provider()