feat: Add AI/ML API provider support

[D1m7asis]

from: https://github.com/gptme/gptme/issues/548

Adds AI/ML API as a new provider (aimlapi) with:

• Provider detection via AIML_API_KEY
• OpenAI-compatible client (https://api.aimlapi.com/v1)
• Dynamic model listing from /v1/models
• Extra headers (HTTP-Referer, X-Title)
• Docs update (config + providers)
• Tests for detection, headers, and provider listing

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How to test

export AIML_API_KEY=<xxxxxx>

gptme "hello" -m aimlapi/openai/gpt-4o-mini
pytest -k aimlapi

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Notes

• No breaking changes to other providers
  • OpenRouter logic is unchanged.
  • Extra headers are now applied to both OpenRouter and AI/ML API.
  • Dynamic model listing, previously only for OpenRouter, now also supports AI/ML API.

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[!IMPORTANT] Adds support for AI/ML API provider with dynamic model listing and extra headers, updating configuration and tests accordingly.

• Behavior:
  • Adds aimlapi as a new provider with detection via AIML_API_KEY in __init__.py and llm_openai.py.
  • Supports dynamic model listing from /v1/models in llm_openai.py.
  • Adds extra headers (HTTP-Referer, X-Title) for aimlapi in llm_openai.py.
• Configuration:
  • Updates config.rst and providers.rst to include AIML_API_KEY.
  • Adds aimlapi to models.py with default and summary model settings.
• Testing:
  • Adds test_llm_aimlapi.py for provider detection, headers, and listing.
  • Updates conftest.py to check for AIML_API_KEY.

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