sessions: remove session deduplication (#317311)

Remove the deduplicationKey concept from ISession and SessionsManagementService.

Previously, local and remote agent-host providers could both expose the same
underlying agent session, causing duplicates in the sidebar. The
deduplicationKey (set to the session URI string) was used to filter these out
in getSessions().

The deduplication is removed entirely. Duplicate sessions from different
providers will now appear as separate entries, which is acceptable given the
rarity of the local+remote overlap scenario.

- Remove deduplicationKey from ISession interface
- Remove deduplicationKey field from AgentHostSessionAdapter
- Remove deduplicateSessions() function and its call in getSessions()
- Remove the now-unused LOCAL_AGENT_HOST_PROVIDER_ID import
- Remove deduplicateSessions test suite

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>