Agent-side changes to support JetBrains integration tests

[steveyegge]

Most of the changes were on the JetBrains/Kotlin side; there isn't much here:

• added a new file scheme, temp://, for jetbrains in-memory integration tests that use a temp filesystem
• added support for an integration test access token passed via env var
• stub for a Mock LLM provider which does nothing yet

Other changes:

• I added an extra VS Code run configuration for connecting to a remote Agent, for debugging.

Test plan

This is part of the test plan for previous commits, as it helps introduce JetBrains integration testing. It complements PR https://github.com/sourcegraph/jetbrains/pull/1291