Fix incorrect context size limits for GitHub Copilot Chat models

Fixes #44909

Problem

The agent panel was displaying incorrect token limits for GitHub Copilot models. Users reported that:

• The agent panel always showed a 128K token limit for all GitHub Copilot models, regardless of their actual context window size
• Claude models (e.g., Claude 3.7 Sonnet, Claude Opus 4.5) were showing ~90K instead of their actual 200K context window
• GPT-4o was showing 110K instead of its actual 128K context window
• Users could continue using models beyond the displayed limit, which worked but was confusing

Root Cause

The max_token_count() method in copilot_chat.rs was returning max_prompt_tokens instead of max_context_window_tokens:

// Before (incorrect)
pub fn max_token_count(&self) -> u64 {
    self.capabilities.limits.max_prompt_tokens
}

GitHub's API returns three different token-related fields:

• max_context_window_tokens: The full context window size (e.g., 200K for Claude 3.7)
• max_prompt_tokens: GitHub's limit for prompt input (e.g., 90K for Claude 3.7)
• max_output_tokens: Maximum output tokens (e.g., 16K)

The max_token_count() method in the LanguageModel trait is expected to return the full context window size — this is consistent with all other providers (Anthropic returns 200K for Claude, OpenAI returns 128K for GPT-4o, etc.).

Solution

Changed max_token_count() to return max_context_window_tokens:

// After (correct)
pub fn max_token_count(&self) -> u64 {
    self.capabilities.limits.max_context_window_tokens as u64
}

Impact

Testing

Added a new test test_max_token_count_returns_context_window_not_prompt_tokens that:

1. Deserializes model JSON with distinct max_context_window_tokens and max_prompt_tokens values
2. Verifies Claude 3.7 Sonnet returns 200,000 (context window), not 90,000 (prompt tokens)
3. Verifies GPT-4o returns 128,000 (context window), not 110,000 (prompt tokens)

All existing tests continue to pass:

running 4 tests
test tests::test_unknown_vendor_resilience ... ok
test tests::test_max_token_count_returns_context_window_not_prompt_tokens ... ok
test tests::test_resilient_model_schema_deserialize ... ok
test result: ok. 4 passed; 0 failed

Release Notes:

• copilot: Fixed incorrect context window size displayed for GitHub Copilot Chat models in the agent panel.