fix: recompute runtime context length across runtime switches

[leavrcn]

Summary

• add a shared helper to resolve runtime _config_context_length
• recompute _config_context_length in switch_model(), _try_activate_fallback(), and _restore_primary_runtime()
• keep top-level model.context_length precedence unchanged
• add focused regression coverage for init precedence, runtime switch, fallback, and restore

Test Plan

• pytest -q tests/run_agent/test_switch_model_context.py tests/run_agent/test_compressor_fallback_update.py tests/run_agent/test_runtime_context_length_refresh.py
• pytest -q tests/run_agent/test_provider_fallback.py tests/run_agent/test_switch_model_fallback_prune.py tests/hermes_cli/test_custom_provider_context_length.py
• pytest -q tests/run_agent/test_switch_model_context.py tests/run_agent/test_compressor_fallback_update.py tests/run_agent/test_runtime_context_length_refresh.py tests/hermes_cli/test_custom_provider_context_length.py tests/run_agent/test_provider_fallback.py tests/run_agent/test_switch_model_fallback_prune.py

Notes

• excluded local-only files from the commit, including .hermes/, docs/, and tests/gateway/test_run_progress_cards.py

[leavrcn]

Closing — superseded by active work under PR #50384. Will reopen if needed.