docs: add FuturMix as OpenAI-compatible AI gateway provider

[FuturMix]

Summary

Add FuturMix as an integration provider in the LangChain documentation. FuturMix is a unified AI gateway that provides access to 22+ models (OpenAI, Anthropic, Google) through a single OpenAI-compatible endpoint with 99.99% SLA.

Since FuturMix exposes an OpenAI-compatible Chat Completions API, it works directly with ChatOpenAI via a custom base_url — no additional package is needed.

Changes

• src/oss/concepts/providers-and-models.mdx — Added FuturMix to the "Routers and proxies" table alongside OpenRouter and LiteLLM
• src/oss/python/integrations/providers/futurmix.mdx — New provider page describing FuturMix and linking to the chat integration
• src/oss/python/integrations/chat/futurmix.mdx — New chat integration page with setup, instantiation, invocation, streaming, and chaining examples using ChatOpenAI with base_url="https://futurmix.ai/v1"
• src/oss/python/integrations/providers/all_providers.mdx — Added FuturMix card in alphabetical order (between Friendli and Galaxia)

Why this belongs here

FuturMix fits the same "Router / Gateway" category as OpenRouter and LiteLLM — it aggregates multiple AI providers behind a single API. The existing ChatOpenAI class works out of the box with FuturMix's OpenAI-compatible endpoint, making this a zero-dependency integration for LangChain users.

Test plan

• Verify all new pages render correctly
• Verify links between provider page and chat integration page work
• Verify the routers and proxies table in providers-and-models.mdx renders correctly
• Verify alphabetical ordering in all_providers.mdx

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[github-actions]

Thanks for opening a docs PR, FuturMix! When it's ready for review, please add the relevant reviewers:

• @npentrel or @lnhsingh (open source)
• @mdrxy (Python integrations)