filesystem: with no directories and no Roots support, the startup error is swallowed and the server stays up with nothing allowed #4992

Problem

The README says that a server started with no allowed directories, connected to a client without Roots support, fails at initialization. The error is thrown in oninitialized, which the SDK does not await, so it becomes an unhandled rejection. The session stays up with an empty allow-list, and every file operation is refused.

Where it is pinned

src/filesystem/__tests__/server-roots.test.ts: "stays connected with nothing allowed when no directories were given", added in #4977 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

The server reports the error to the client and exits (or otherwise fails visibly), as the README describes.

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.