* feat(session-manager): allow supplying custom session ID in newSession()
Add optional `id` field to `NewSessionOptions`. When provided, this ID
is used as the session ID instead of generating a random UUID. Existing
callers are unaffected since the field is optional and falls back to
`randomUUID()`.
Closes#2097
* test(session-manager): add tests for custom session ID in newSession()
Verify that newSession() uses the provided id when supplied, falls back
to randomUUID() when omitted, and includes the custom id in the session
header.
