Assistant: Provide a way for R packages to register language model tools

The tools btw and acquaint separate the concepts of "LLM tools" (btw) and "an MCP server that provides the tools to an LLM" (aquaint).

Positron's built-in tool calls for R are always going to be limited (and probably outdated) compared to what third parties can build for their own packages. We can empower the community (and the R team here) to build very rich tooling that we don't have the capacity to build in the Positron core. What if we built a way for R packages to register LLM tools directly via something like rstudioapi? It would work something like this:

sequenceDiagram
R Package ->> R Kernel: rstudioapi::registerLanguageModelTool
R Kernel ->> R Extension: RegisterTool comm message
R Extension ->> Positron: vscode.lm.registerTool
Positron ->> R Extension: Invoke Tool
R Extension ->> R Kernel: InvokeTool comm message
R Kernel ->> R Package: Run callback
R Package ->> R Kernel: Callback result
R Kernel ->> R Extension: ToolResult comm message
R Extension ->> Positron: Callback result

Some implementation notes:

• Lifecycle management needed; tools would need to be re-registered after an extension host restart, on reload, unregistered when session ends, etc. On a reload, how would we remember what tools were registered and now need re-registration? Could be the responsibility of the extension, or we could maintain a list on the Positron side.
• Care is needed to avoid multiple R sessions registering multiple instances of the same tool. We would probably want a Positron API that looks like vscode.lm.registerTool but registers a session-scoped tool that is used only when the given session is focused/active.

No files or tests are named. Start by tracing the R package, R Kernel, R Extension, and Positron message flow around rstudioapi and vscode.lm; completion should cover registration, invocation, session scoping, and behavior across extension restarts, reloads, and session ends.