fix: correct pricing calculation for providers using $/token format

[chrisnestrud]

Problem:

Commit c6a90d5 added division by DEFAULT_TOKEN_PRICE_RATIO (1000000) to convert pricing from $/M to $/token format. However, this was applied to ALL providers, but different providers use different pricing formats:

• Synthetic and OpenRouter: Already return pricing in $/token format (e.g., "$0.00000055" per token)
• LiteLLM's model_prices_and_context_window.json: Uses $/M format (e.g., "2.5e-06" = $2.5/M tokens)

This caused synthetic provider pricing to be divided by 1,000,000 twice, making costs appear 1,000,000x too small. For example:

• synthetic/hf:zai-org/GLM-4.7: $0.00000055/token became $0.00000000000055/token
• User reported: 17k sent, 10 received tokens showed cost of $0.0000000096

Solution:

Added a heuristic in ModelProviderManager._record_to_info() to detect pricing format before applying conversion:

• If cost >= 0.001: Treat as $/M format, divide by 1,000,000
• If cost < 0.001: Treat as $/token format, no division

This threshold (0.001) was chosen because:

• $/M pricing is typically >= $0.001/M (e.g., $1.0/M, $2.5/M)
• $/token pricing is typically < $0.001/token (e.g., $0.00000055/token)

Changes:

1. aider/helpers/model_providers.py:

   • Added _normalize_cost() helper function to intelligently handle both formats
   • Replaced unconditional division with format-aware normalization

2. tests/basic/test_model_provider_manager.py:

   • Added test_pricing_normalization_detects_token_format: Verifies $/token pricing is not divided
   • Added test_pricing_normalization_detects_million_format: Verifies $/M pricing is converted correctly

Impact:

• Synthetic provider models now display correct pricing
• Existing $/M format providers continue to work correctly
• No breaking changes to API or behavior

Co-authored-by: aider-ce (synthetic/hf:zai-org/GLM-4.7)