xAI /v1/models does not return context_length metadata, so Hermes
probes down to the 128k default whenever a user configures a custom
provider pointing at https://api.x.ai/v1. This forces every xAI user
to manually override model.context_length in config.yaml (2M for
Grok 4.20 / 4.1-fast / 4-fast) or lose most of the usable context
window.

Add DEFAULT_CONTEXT_LENGTHS entries for the Grok family so the
fallback lookup returns the correct value via substring matching.
Values sourced from models.dev (2026-04) and cross-checked against
the xAI /v1/models listing:

  - grok-4.20-*          2,000,000  (reasoning, non-reasoning, multi-agent)
  - grok-4-1-fast-*      2,000,000
  - grok-4-fast-*        2,000,000
  - grok-4 / grok-4-0709   256,000
  - grok-code-fast-1       256,000
  - grok-3*                131,072
  - grok-2 / latest        131,072
  - grok-2-vision*           8,192
  - grok (catch-all)       131,072

Keys are ordered longest-first so that specific variants match before
the catch-all, consistent with the existing Claude/Gemma/MiniMax entries.

Add TestDefaultContextLengths.test_grok_models_context_lengths and
test_grok_substring_matching to pin the values and verify the full
lookup path. All 77 tests in test_model_metadata.py pass.