cody: refactor for unit testing

[novoselrok]

This PR refactors parts of the extension to make it unit testable, specifically the Transcript class. This involved making the Transcript class and its dependencies independent of the VSCode API. I replaced specific implementations with interfaces where possible, allowing us to mock any dependencies we need. I added a single unit test as a proof of concept, and more will be coming in subsequent PRs.

I recommend that you review commit by commit, as I tried to group the relevant changes as much as possible.

Test plan

• Run pnpm run build && pnpm run test:unit && pnpm run test:integration, tests should be green.

App preview:

• Web

Check out the client app preview documentation to learn more.

[dominiccooney]

Looks great.

Later we may want a "context" object, with each client taking one parameter which is the intersection of the parts of the context it needs, to cut down on stuttering in the parameter lists.