Arize Phoenix v20.4.0 adds an in-process MCP toolset and splits custom model providers onto their own endpoint

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Released 2026-08-26, Phoenix v20.4.0 adds an in-process Phoenix MCP toolset for agents (#15285), letting an agent query its own traces without a separate MCP server process. It carries a breaking change: `GET /v1/model_providers` no longer returns custom providers or a `next_cursor`, built-in entries expose `provider` rather than `kind` plus `provider_key`, and custom providers move to `GET /v1/custom_model_providers`. The endpoint is unreleased so no published client breaks.