fix: unify context length overrides for compression runtimes

[slicenferqin]

Summary

• unify context_length override resolution for compression-related runtimes
• reuse custom_providers[].models[*].context_length across init, switch, fallback, and compression feasibility checks
• add regression coverage for override refresh/clear paths

Closes #8785.

Problem

Custom provider context length overrides were not applied consistently.

The main runtime could honor custom_providers[].models[*].context_length, but other paths could still re-probe provider metadata and fall back to 128000 when /models did not expose a context field. This caused spurious compression feasibility warnings and inconsistent runtime state.

Changes

• add a shared override resolver in AIAgent
• apply it during initial compressor setup
• apply it when switching runtimes
• apply it during compression feasibility checks for auxiliary compression models
• apply it when activating fallback runtimes and when restoring the primary runtime snapshot
• add regression tests for:
  • init uses custom provider override
  • switch refreshes override for new runtime
  • switch clears stale override when the new runtime has none
  • compression feasibility checks use the same override

Testing

• uv run --extra dev pytest tests/run_agent/test_switch_model_context.py tests/run_agent/test_primary_runtime_restore.py tests/agent/test_model_metadata.py -q -n 0

[slicenferqin]

Correction to my previous comment: follow-up enhancement request filed in #8810 to add an explicit auxiliary.compression.context_length config field. This PR fixes the current inconsistency, while #8810 would make compression runtime context configuration first-class and less dependent on provider metadata for custom endpoints.

[teknium1]

Thanks for the focused regression coverage. This automated hermes-sweeper review found that current main already provides the requested behavior.

• Commit 125de02056eab84362fc91f57bd7041a19860b22 added canonical custom_providers[].models[*].context_length resolution across startup and live /model switching; current implementation is in agent/agent_init.py:1653 and agent/agent_runtime_helpers.py:1990.
• Fallback activation resolves the fallback runtime with custom_providers before updating the compressor in agent/chat_completion_helpers.py:1511 (the original fallback fix is 46451528a50ad1adff2591650e9f0ccda561cd26).
• Compression feasibility passes both the explicit auxiliary override and custom providers to the shared resolver in agent/conversation_compression.py:236.
• The follow-up noted in the discussion, explicit auxiliary.compression.context_length, is present in agent/agent_init.py:1584 and covered by tests/run_agent/test_compression_feasibility.py:176.

The original run_agent.py targets have since been extracted into these modules, so this PR is now redundant.