fix(agent): pass custom providers to aux context checks

[LeonSGP43]

What does this PR do?

Fixes auxiliary compression feasibility checks so they pass through configured custom providers when checking the available context window, which keeps custom-model auxiliary setups from being misclassified.

Related Issue

Fixes #21947

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• Persisted custom providers during agent initialization in run_agent.py
• Passed custom providers into the compression feasibility check
• Added regression coverage in tests/run_agent/test_compression_feasibility.py

How to Test

1. Run uv run --frozen pytest -q -o addopts='' tests/run_agent/test_compression_feasibility.py
2. Run uv run --frozen ruff check run_agent.py tests/run_agent/test_compression_feasibility.py
3. Confirm both commands pass

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15.x

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

• uv run --frozen pytest -q -o addopts='' tests/run_agent/test_compression_feasibility.py -> 17 passed

[alt-glitch]

Duplicate of #13540 (and competing PRs #18877, #20724). All pass custom_providers to compression feasibility check. Fixes #21947/#12977/#20608.

[teknium1]

Automated hermes-sweeper review: this is now implemented on current main.

Evidence:

• agent/agent_init.py:1398 loads compatible custom providers from config, and agent/agent_init.py:1407 stores them on agent._custom_providers for the compression feasibility path.
• agent/conversation_compression.py:136 calls get_model_context_length(...) for the auxiliary compression model, and agent/conversation_compression.py:145 passes custom_providers=agent._custom_providers.
• The existing maintainer discussion on this PR noted it duplicates fix(agent): propagate custom provider context_length to compression feasibility check #13540 / fix(context): pass custom_providers in aux compression and fallback paths #18877 / fix(run_agent): pass custom_providers to compression feasibility check (#20608) #20724, all targeting the same custom-provider compression feasibility bug.

Thanks for the fix; the behavior is covered by the current main implementation.