perf(session): reduce startup and historical session overhead

[limityan]

Summary

• Adds a lightweight historical-session restore path so first opening an existing session can paint persisted chat turns without eagerly restoring the full runtime context.
• Keeps compatibility fallbacks for older desktop/remote endpoints: restore_session_view falls back to restore_session_with_turns, then to the legacy restore_session + load_session_turns sequence; get_configs falls back to per-key get_config.
• Reduces normal dialog-turn save overhead by updating session metadata incrementally, with a per-session metadata update lock and full-scan fallback for inconsistent storage.
• Defers snapshot refresh work while historical session data is metadata-only/hydrating or backend context restore is still pending, and dedupes several startup/background initialization paths.

Behavior

• Historical session open now prioritizes visible UI readiness: persisted turns are loaded and rendered first, and backend coordinator context is restored before the next action that requires it.
• Visible tool result payloads are preserved for UI/history display; assistant-only tool result text is trimmed from the session-view response to reduce first-load payload size.
• First-turn sessions that cannot restore backend metadata still keep the old recreate fallback, while multi-turn historical sessions continue to fail loudly instead of silently losing context.
• Remote scenarios keep passing connection/workspace context through the service APIs, and unsupported restore-command caching is scoped by remote identity so an older remote endpoint does not disable the optimized path for local or newer endpoints in the same runtime.

Performance Benefits

• Startup AI config reload is batched from multiple IPC calls into one get_configs call on supported builds.
• First historical-session open avoids duplicate turn loading and eager runtime snapshot restoration on the common path.
• Dialog-turn save metadata refresh changes from scanning all turn files to reading only the current turn's previous persisted file on the normal path.
• Snapshot refresh avoids cold-start contention while a historical chat is not ready or still has pending backend context restore.
• Startup/session diagnostics are thresholded or capped; session response size estimation is bounded at 2MB to avoid tracing itself becoming a hot-path cost on very large sessions.

Risks / Watch Points

• The deferred historical-session context state adds a pending-restore transition; if multi-turn context restoration fails, sending or context-dependent actions still surface an error rather than silently creating a partial session.
• Incremental metadata updates rely on serialized per-session metadata writes; tests cover concurrent updates, and storage inconsistency still falls back to a full metadata scan.
• New diagnostics are thresholded/debug-oriented, but debug sessions should still watch log volume around very large sessions.
• Mixed old/new desktop or remote endpoints rely on fallback paths, so the compatibility behavior should be kept covered while these endpoints roll forward.

Independent Audit Adjustments

• Reduced frontend session response trace estimation from 32MB to 2MB to avoid adding measurable first-open CPU work for ultra-long sessions.
• Extended snapshot refresh gating to contextRestoreState=pending, avoiding snapshot manager initialization while historical context is intentionally deferred.
• Scoped unsupported restore-command memoization by remote identity to avoid an old remote endpoint degrading local/newer endpoints in the same app runtime.

Verification

• git diff --check upstream/main..HEAD
• rustfmt --edition 2021 --check src/crates/core/src/agentic/persistence/manager.rs
• pnpm run type-check:web
• pnpm run lint:web
• pnpm exec vitest run src/flow_chat/services/flow-chat-manager/SessionModule.test.ts
• pnpm exec vitest run src/infrastructure/api/service-api/ApiClient.test.ts src/tools/snapshot_system/hooks/snapshotRefreshPolicy.test.ts src/flow_chat/store/FlowChatStore.test.ts
• pnpm exec vitest run --maxWorkers=1
• cargo test -p bitfun-core save_dialog_turn_updates_metadata_without_scanning_unrelated_turn_files -- --nocapture
• cargo test -p bitfun-core concurrent_dialog_turn_saves_keep_metadata_counts_consistent -- --nocapture
• cargo check -p bitfun-desktop