What does this PR do?

Fixes gateway session info / status context-length detection for models defined under custom_providers.

Previously _format_session_info() only respected model.context_length at the top level, so custom provider models with per-model context_length metadata fell back to the default 128K display/hint path.

This PR adds a shared lookup helper for custom_providers per-model context lengths and uses it in both session-info formatting and gateway session hygiene fallback logic.

Related Issue

Fixes #5089

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• gateway/run.py
  • added a helper to resolve context_length from custom_providers
  • taught _format_session_info() to use that helper when top-level model.context_length is absent
  • reused the same helper in gateway hygiene fallback logic
  • supports both custom_providers[].models as a dict map and as a list of {name, context_length} entries
• tests/gateway/test_session_info.py
  • added regression coverage for custom_providers list entries carrying context_length

How to Test

1. Configure a custom provider like:

   model:
     default: my-huge-model
     provider: custom
     base_url: https://my-gateway.io/v1
   
   custom_providers:
     - name: my-custom
       base_url: https://my-gateway.io/v1
       models:
         - name: my-huge-model
           context_length: 1100000

2. Open the gateway status/session info view.
3. Verify the context display uses the configured custom-provider value instead of falling back to 128K.

Or run:

python -m pytest tests/gateway/test_session_info.py -q -n0

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run relevant pytest coverage locally
• I've added tests for my changes
• I've tested on my platform:
  • macOS

Documentation & Housekeeping

• I've considered cross-platform impact
• N/A — no docs/config schema changes
• N/A — no tool schema changes