fix(sessions): use non-empty path for unknown-workspace fallback URI to prevent joinPath crashes (#310777)

[maruthang]

What

Replace the repo-less session fallback URI URI.parse('unknown:') with URI.from({ scheme: 'unknown', path: '/' }) so downstream workspace-folder operations don't explode on a path-less URI.

Why

URI.parse('unknown:') yields a URI with an empty path. URI.joinPath throws cannot call joinPath on URI without path for such URIs, so any code iterating workspace folders and calling folder.toResource(...) / URI.joinPath(folder.uri, ...) crashed — fanning out into ~13 telemetry buckets (#310777, #310752, #310751, #310369, #310370, #310368, #310364, #310365, #310366, #310357, #310180, #308827).

Snippets service, workspace extensions config, preferences editor menu, and compareAndDeleteFolderConfiguration all hit this path for Copilot agent sessions without repository metadata.

How

URI.from({ scheme: 'unknown', path: '/' }) produces a URI whose path is non-empty, so URI.joinPath and friends behave correctly. Fallback is only taken when repoUri === undefined; sessions with real repo URIs are unaffected.

This was the approach flagged (but deferred) by @aeschli when landing PR #309777.

Test plan

New test in copilotChatSessionsProvider.test.ts: construct a session with metadata: undefined, read session.workspace.get(), and assert the fallback URI has a non-empty path and that URI.joinPath(fallbackUri, '.vscode', 'settings.json') does not throw (both were broken pre-fix).

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Closes the root cause for the following telemetry buckets: #310777, #310752, #310751, #310370, #310368, #310364, #310365, #310366, #310357, #310180, #308827.

(Does NOT close the separate compareAndDeleteFolderConfiguration race-condition bug tracked in #310369 — that's a distinct fix.)

Fixes #310777

[aeschli]

Thanks a lot, it's fixed by #311319 I used your test case