Summary

• Defer parsing Docker-only JSON terminal env vars until the Docker backend is selected.
• Use safe Docker defaults for local/ssh so stale malformed Docker config cannot break subprocess tools.
• Keep Docker backend validation intact with a clear TERMINAL_DOCKER_VOLUMES error for bad JSON.

Why

TERMINAL_DOCKER_VOLUMES=None and similar malformed Docker-only env/config values were parsed eagerly inside terminal config loading, which could crash the default local backend before Docker was ever used.

Changes

• Gate Docker JSON parsing on TERMINAL_ENV=docker.
• Gate generic container resource parsing to container-style backends.
• Add regression coverage for local, ssh, and Docker backend behavior.

Validation

• python -m ruff check tools/terminal_tool.py tests/tools/test_terminal_tool.py
• python -m pytest tests/tools/test_terminal_tool.py tests/tools/test_terminal_config_env_sync.py -o 'addopts=' -q

Scope

Focused terminal config parsing regression fix only.

Closes #42725