Copilot's /models endpoint exposes token limits under capabilities.limits.
max_prompt_tokens is the actual prompt capacity (what OMP calls contextWindow),
while max_context_window_tokens is the total window (prompt + output budget).
Using the latter inflates contextWindow, which breaks compaction thresholds,
overflow detection, and context promotion.
Three changes:
1. mapModel reference selection: always prefer the Copilot-specific bundled
reference over the global cross-provider reference. Copilot imposes its
own limits that are strictly lower than native provider limits.
2. mapModel contextWindow chain: remove max_context_window_tokens from the
fallback. New chain: context_length -> max_prompt_tokens -> reference.
3. generate-models: stop overwriting contextWindow/maxTokens in
applyGlobalModelsDevFallback. These are provider-specific and should not
be replaced with cross-provider models.dev global references.
Also fixes bundled values: github-copilot/gpt-5.4 (400k -> 272k) and
github-copilot/gpt-5.2 (264k -> 128k) to match live API max_prompt_tokens.
Refs: #225, #226
