fix config and stream timeout validation

[hamb1y]

Scope

Config and environment validation

Issues Fixed

• Fixes Bug: invalid NANOBOT_STREAM_IDLE_TIMEOUT_S crashes streaming setup #4065: invalid NANOBOT_STREAM_IDLE_TIMEOUT_S values fall back to safe defaults across streaming providers.
• Fixes Bug: invalid config silently falls back to defaults #4067: invalid config parse or schema errors now fail closed instead of silently continuing with defaults.

Key Files

• nanobot/config/loader.py
• nanobot/utils/helpers.py
• nanobot/providers/anthropic_provider.py
• nanobot/providers/bedrock_provider.py
• nanobot/providers/openai_codex_provider.py
• nanobot/providers/openai_compat_provider.py
• tests/config/test_env_interpolation.py
• tests/providers/test_custom_provider.py

Validation

• uv run pytest tests/config/test_env_interpolation.py::test_load_config_invalid_json_fails_closed tests/providers/test_custom_provider.py::test_invalid_stream_idle_timeout_falls_back_to_default tests/providers/test_anthropic_stream_idle.py tests/providers/test_openai_codex_provider.py::test_codex_request_honors_stream_idle_timeout_env -q