tests: make real subagent interrupt test hermetic

[shimotmr]

Summary

• make test_real_interrupt_subagent hermetic against leaked terminal backend env
• isolate the test from unrelated preflight checks that can block child construction before run_conversation() starts

Root cause

This test is intended to verify delegate interrupt propagation, but in suite runs it could be influenced by:

• leaked TERMINAL_ENV / TERMINAL_MODAL_MODE
• _build_system_prompt
• _check_compression_model_feasibility() and its auxiliary/provider preflight path

That allowed the child to get stuck during construction / pre-conversation setup instead of exercising the interrupt path the test claims to cover.

Changes

• force TERMINAL_ENV=local in setUp() and restore env in tearDown()
• patch _build_system_prompt
• patch _check_compression_model_feasibility() so the test stays focused on interrupt propagation rather than auxiliary compression/provider preflight behavior

Verification

• pytest -n0 tests/tools/test_terminal_requirements.py::test_modal_backend_direct_mode_does_not_fall_back_to_managed tests/run_agent/test_real_interrupt_subagent.py::TestRealSubagentInterrupt::test_interrupt_child_during_api_call -vv -s
• pytest tests/run_agent/test_real_interrupt_subagent.py::TestRealSubagentInterrupt::test_interrupt_child_during_api_call -vv -s
• ./scripts/run_tests.sh tests/run_agent/test_real_interrupt_subagent.py tests/tools/test_terminal_requirements.py --maxfail=1 -q

Notes

This fixes the test/harness hermeticity issue. There may still be a production-level follow-up worth discussing around whether AIAgent.__init__ should do auxiliary compression feasibility checks eagerly for child agents.