Fix: memory-context cache survives invalidation, leaking stale snapshots

[mimeding]

Summary

Why this matters (business)

Agents are supposed to react immediately when a user edits their memory (changing a fact, adding a user override, switching tool availability). With this bug, changes a user just made could silently fail to take effect for up to 10 seconds after a chat window event — the agent answers as if the edit never happened. That's exactly the kind of "did it work?" UX issue that erodes trust in local memory, and it can subtly regress the chat/tool partition fix that landed in osaurus-ai#877.

What's wrong (technical)

MemoryContextAssembler caches assembled context with a composite key:

    private static func cacheKey(agentId: String, toolsAvailable: Bool) -> String {
        "\(agentId)|tools=\(toolsAvailable ? 1 : 0)"
    }

…but invalidateCache(agentId:) removed entries using only the bare agentId:

    public func invalidateCache(agentId: String? = nil) {
        if let agentId {
            cache.removeValue(forKey: agentId)
        } else {

The key never matched, so the call was a no-op. Callers that depend on this contract — notably ChatWindowManager on window close — could not actually force a fresh assembly:

// Drop any 10-second-TTL memory context snapshot so a freshly
// opened window for the same agent rebuilds from current state.
// Without this, a user who edits memory in window B and closes
// window A could briefly see the stale A-era assembly on the
// next compose pass.
await MemoryContextAssembler.shared.invalidateCache(agentId: aid.uuidString)

The comment above describes the exact symptom the bug allows.

Fix

Evict both tools=0 and tools=1 partitions for the given agentId. The global (nil) and per-agent invalidation now both behave as advertised.

Adds two test-only internal helpers (_seedCache, _hasCachedEntry) so the eviction contract can be verified without standing up MemoryDatabase, MemorySearchService, and an embedder just to populate the cache through the normal path.

Changes

• Behavior change (fixes a latent bug in cache invalidation; no API change)
• UI change
• Refactor / chore
• Tests (regression test for cache-key mismatch + agent-scoped invalidation)
• Docs

Test Plan

1. cd Packages/OsaurusCore && swift test --filter MemoryContextAssemblerTests
2. Manually: open two chat windows for the same agent, edit a user override in one, close the other, send a message in the first — the new override should appear in the assembled context on the very next turn (previously could be deferred by up to 10s).

Checklist

• I have read CONTRIBUTING.md
• I added/updated tests where reasonable
• I updated docs/README as needed (no user-facing API change)
• I verified build on macOS with Xcode 16.4+ (authored in a Linux CI sandbox; please run the Xcode build before merging)