Summary

• reuse the same custom provider context_length resolution for both main-model startup and auxiliary compression feasibility checks
• pass custom_providers.models..context_length through when compression falls back to the main custom provider model
• avoid duplicate invalid-context warnings when the same bad custom provider entry is touched by both startup paths
• add a regression covering the auxiliary fallback case

Testing

• python3 -m pytest -o addopts= tests/run_agent/test_compression_feasibility.py -k "custom_provider_context_length or passes_config_context_length or ignores_invalid_context_length"
• python3 -m pytest -o addopts= tests/run_agent/test_compression_feasibility.py
• python3 -m pytest -o addopts= tests/run_agent/test_invalid_context_length_warning.py

Closes #12977