fix(resolver): ignore whitespace-only OPENSPACE_LLM overrides

[Gujiassh]

Summary

• treat whitespace-only OPENSPACE_LLM env values as empty overrides
• avoid skipping host config fallback when OPENSPACE_LLM_API_KEY/OPENSPACE_LLM_API_BASE are only spaces
• normalize OPENSPACE_CONFIG_PATH by trimming whitespace and returning None when empty
• add resolver regression tests for whitespace handling

Validation

• python3 -m unittest tests/test_host_detection_resolver.py