R: Modernize the session test architecture

[eitsupi]

Background

While working on #1739, several weaknesses in the current R/session tests became apparent.

For example:

• the bundled tinytest suite was not run in CI;
• some existing tests had already become stale;
• session E2E tests rely heavily on Terminal.sendText() plus fixed sleeps/timeouts;
• failures in R evaluation, callbacks, IPC, and VS Code handling often all appear as the same timeout;
• tinytests do not reproduce vscode-R's actual R startup path through R_PROFILE_USER;
• some transport tests use pipe primitives that do not exactly match production Unix sockets / Windows named pipes.

Proposal

Modernize the tests incrementally, roughly separating them into:

1. R-level tests — runtime lifecycle, hooks, options, Data Viewer state, cleanup.
2. sess transport tests — real socket/named-pipe request/response, disconnect and reconnect behavior.
3. interactive R E2E — launch a real R session and deterministically observe startup, command completion, task callbacks, and prompt return.
4. VS Code E2E — keep a smaller set of tests for terminal creation, workspace updates, plots, View(), etc.

In particular, we should reduce sendText() + sleep + waitFor() style synchronization and make failures identify which layer actually failed.

This does not need to be a large rewrite. The goal is to improve the suite incrementally so it is more deterministic, closer to real usage, and easier to debug and extend.

[grantmcdermott]

I also notcied/realised the missing CI issue when I was working on #1743. I think we should fix this first, before your items 1-4.

[eitsupi]

Just to be clear, the issue I'm discussing here is session testing.