fix/chat: Send workspace root as the expected URI, not a path

[dominiccooney]

JetBrains was initializing Agent passing a path instead of a workspace root URI. This is fine on UNIX-style systems, because the string /foo/bar is interpreted as a file URI. But on Windows it meant we were passing paths like c:/Users/bob/project (yes... we convert the path separator to a forward slash etc.) and Agent was treating this as /Users/bob/project which is wrong.

The fix is simply to pass a URI instead of a file.

In addition, normalizeUriOrPath was lowercasing drive letters, but in the URI case only if the URI was file://C:/foo; this fixes it to also handle file:///C:/foo (note the extra slash.)

Note, Extension handling often? converts these URLs from file:///c:/foo to file:///c%3a/foo and does prefix checks on URIs as strings or paths, so we should consider canonicalizing colons in file URLs too, but that will take more care and attention. This change is enough to unbreak Prompt Library mentions on Windows.

Test plan

On Windows,

• Run with prompts as commands enabled (currently set up on s2)
• Open a file, right click, Cody, Explain Code.
• Check that the filename has a @file.txt presentation on a mention slab, instead of @C:\full\path\to\file.txt without a mention slab.

Changelog

• Agent is now configured with an accurate workspace on root on Windows, like file:///c:/Users/Bob/project not /Users/bob/project.
• On Windows, at-mentions for explain, document, smell, etc. display as tiles with short names.

[pkukielka]

LGTM.
That said, we really need to get rid of those paths-or-uris-as-strings everywhere in the protocol.