QVAC-16480 fix: fix kv-cache save race condition in tool-calling completions

[gusttav-lang]

🎯 What problem does this PR solve?

• Multi-turn tool-calling completions crash with "Cannot set new job: a job is already set or being processed" on Linux AppImage/Snap builds
• The crash is a race condition: the KV-cache save from iteration N is still in-flight when the next completion call begins, because _hasActiveResponse was never cleared
• Only reproduces in packaged Linux environments (FUSE-mounted squashfs) where I/O latency widens the race window; dev mode timing masks the bug

📝 How does it solve it?

• Include the cache file path as the first session message in the save command array — without it the native addon silently rejects the save
• Await saveResp.await() so _hasActiveResponse is cleared (via its .finally()) before the generator yields stats and the next completion() call begins
• Move the model.run() save call inside the if (sessionMsg?.content) guard since saving without a valid cache path always fails

🐛 Bug screenshot

🧪 How was it tested?

• Reproduced the crash on Linux AppImage with Skill tool calls (e.g. weather Skill), which always triggered the race condition; confirmed the error no longer occurs after the fix
• Verified MCP tool calls (which were not affected) still work correctly
• Verified single-turn and multi-turn completions with KV-cache enabled still work correctly on dev mode (bun run dev)

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PR checks failed

[NamelsKing]

/review

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (1/1)
- 1 Team Lead OR Management approval ✅ (2/1)

**Bypass rule:** Triggered (2+ Team Lead approvals (Tier 1 exception)). This PR is approved regardless of tier.

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