tool_context() equivalent: per-conversation store and tool-calling context

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ellmer recently added tool_context() plus a per-conversation chat$store environment (tidyverse/ellmer#1023, part of tidyverse/ellmer#871). Tools can use it to access per-conversation state without globals, and it survives clone(deep = TRUE).

chatlas has no equivalent: there's no contextvar-based way for a running tool to access the current chat/tool-call context, and no per-chat mutable store.

Possible shape on the Python side: a tool_context() accessor backed by a contextvars.ContextVar set around tool invocation, plus a chat.store dict (or similar) that tools can read/write.