Windows drive letters are case-insensitive and clients may send either
`file:///C:/foo` (RFC 8089) or `file:///c:/foo` for the same file.
`uris.from_fs_path` lower-cases the drive letter, but `Workspace._docs`
stored URIs verbatim, so the two spellings became distinct dictionary
keys. A document opened under one spelling was then invisible under the
other, and lookups silently fell back to reading stale content from disk
instead of the in-memory version.

Add `uris.normalize` and use it for every `_docs` key, so a document is
found regardless of the drive letter case the client used. Non-drive
paths are unchanged.

Workspace._docs is now keyed by uris.normalize(doc_uri), so on Windows a
client-sent 'file:///C:/...' URI is stored as 'file:///c:/...'. The
multiple-workspaces tests inspect _docs directly with the raw message
URI, which no longer matches on Windows. Normalize the expected key in
those assertions; behaviour on other platforms is unchanged.