fix: refresh tool state during session sync

[hsteude]

Summary

• Replace the stale part-count session sync merge with a part-aware merge for loaded messages.
• Allow full session sync to advance same-ID tool parts from pending/running to newer states while preserving newer SSE tool updates.
• Preserve SSE-only messages and parts during full sync.

Testing

• bun test tests/sync.test.ts
• bun run build
• bun run typecheck fails on existing project-wide TypeScript errors outside this change: src/app.tsx, src/context/server.tsx, src/context/theme.tsx, src/entry.tsx, src/pages/layout.tsx, and ../shared/proxy.ts.

Closes #379
Supersedes #381

[Copilot]

Pull request overview

This PR updates session synchronization so full sync can refresh stale tool part states without discarding newer SSE-only updates.

Changes:

• Adds part-aware message merging for session sync.
• Ranks tool states to prefer newer progress while preserving newer local SSE updates.
• Adds tests for stale pending tools, newer SSE tool updates, and SSE-only message/part preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
app-prefixable/src/context/sync.tsx	Introduces mergeSessionMessages and uses it during session sync.
app-prefixable/tests/sync.test.ts	Adds coverage for tool-state merge behavior and SSE preservation.

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