fix(session): synchronize the engine after foreign session import

[AdityaVG13]

Summary

Follow-up to #6367 (branch side closed by ffca963f9; this PR closes nothing).

/resume <file> and inline-JSON imports install a whole new conversation —
new session id plus transcript — but return a message-only result, so the
engine stays on the previous session's history and the next turn runs
against the wrong transcript. This carries the existing SessionSyncPayload
on ResumeImportReceipt (same shape as the /branch outcome) and maps it
to the SyncSession action in both import arms, reusing the shared
sync_session_action mapper — no new plumbing.

Also adds the missing unit test for rebranch_active_messages_stamped:
shared prefix keeps ids and stamps, the new suffix keeps live stamps, and
the replaced tail survives as a sibling branch.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked (warning-free under the CI allow list)
• cargo test --workspace --all-features --locked — see note below

Targeted suites, all green on the remote worker (Spark):
session::resume:: 6 passed, stamped_rebranch 1 passed,
codewhale-command-contract --lib 59 passed,
lifecycle_portable_tests 8 passed.

Full-workspace note: the workspace run shows 8 failures, all in files this
diff does not touch, and all attributable to main-side churn or the remote
worker's headless environment:
route::export::golden_providers_export_matches_registry (golden stamped
pre-0.10.0; release prep moved the version),
unknown_model_under_zai_never_infers_deepseek_provider_authority
(provider-authority; zero references to the touched types),
clipboard_is_default_and_stdout_is_explicit and
command_clipboard_export_preserves_structure_and_redacts_secrets
(clipboard backend on a headless worker),
explicit_workspace_config_selects_its_route_without_locking_user_global
(area churned by the session-bound account-model feat on main),
system_prompt_prefix_never_leaks_private_content,
separate_predispatch_crashes_on_one_run_get_distinct_recovery_turn_ids,
launch_mcp_summary_opens_manager_by_click_and_keyboard (PTY).
The app-server crate was excluded on the rerun after its deterministic
failure so the remaining suites could execute; 46 suites passed.

Checklist

• This PR adds a new layer/module/abstraction — it names or deletes the layer it replaces (N/A: single facet field, existing mapper)
• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes (N/A: no UI change)
• Harvested/co-authored credit uses a GitHub numeric noreply address (N/A: no harvested code)

No-Issue: Follow-up to the already closed #6367; synchronizes foreign session imports without closing another issue.