fix: Fix inconsistent workspace handling in agent

[fkling]

While trying to adapt #6947 to the changes from #6909, I noticed that workspace handling is inconsistent in the agent.
In particular:

• The agent accepts a workspaceRootUri upon initialization and stores it on AgentWorkspaceDocuments. This value
  eventually ends up in workspaceFolders.
• We support workspaceFolder/didChange, which updates workspaceFolders but doesn't affect workspaceRootUri in
  any way.
• getWorkspaceFolder and asRelativePath ignore workspaces and work directly with workspaceRootUri

So it seems that any client that wants to use workspaceFolder/didChange will eventually end up with an inconsistent state.

This PR aims to fix that. It removes workspaceRootUri and updates all places that used it to work with workspaceFolders instead.
asAbsolutePath was updated to use the extension URI instead, which seems more in line with what the purpose of that method is (though it doesn't seem like we use it anywhere).
workspaceFolders is still initialized from the client info, directly instead of going through workspaceDocuments.

Test plan

I've tested this manually in the context of #6947 and https://github.com/sourcegraph/sourcegraph/pull/3333. The following secanarios work as expected:

• On the chat page, remote repo mentions work (i.e. they appear in the mentions menu). No workspace is set.
• When opening the cody sidebar on a repo page, the workspace is updated (deduced by seeing the Current repository at-mentions entry point to the current repository, as well the default file filtering logic only suggest files from the current repository)
• With the cody sidebar open, navigate directory to a different repository via fuzzy finder. As above, the menu entry and the
  file search correctly resolve to the new repository.

Note

What I've not been able to successfully test is the Rules provider. But I suspect that there might be unrelated issues
with it, because the network requests it made in the web demo resulted in 428 errors, saying that the X-Requested-Width header was missing.

[fkling]

This change is part of the following stack:

• fix: Fix inconsistent workspace handling in agent #7160 ◀
  • feat: Improve warm cody web startup time #6947

_{Change managed by git-spice.}

[fkling]

Is @sourcegraph/cody-agent the right team to tag for review on this? "Agent" is appearing everywhere these days 😅