fix(model-core): add Qwen heuristic variants

[Hungdoan565]

Summary:

• Completes the existing Qwen heuristic family entry by adding fallback variants.
• Makes custom/local Qwen model IDs resolve as heuristic-backed instead of variant-unknown.
• Keeps provider-declared Qwen variants such as xhigh/max/off taking precedence over heuristic fallback.

Verification:

• bun test packages\model-core\src
• bun run typecheck (from packages\model-core)
• git diff --check
• git diff --cached --check

QA: not run; pure Core model capability logic only, no OpenCode/Codex adapter wiring changed.

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Summary by cubic

Add Qwen heuristic variants and enable heuristic fallback for custom/local Qwen model IDs, preventing variant-unknown resolution. Provider-declared Qwen variants still override the fallback.

• Bug Fixes
  • Added ["low", "medium", "high"] variants to the Qwen family in packages/model-core.
  • Heuristic-backed resolution now applies to custom Qwen IDs; provider variants (e.g., xhigh/max/off) take precedence.
  • Expanded tests in model-capabilities.test.ts and model-settings-compatibility.test.ts to cover both behaviors.

^{Written for commit 3ca30f8. Summary will update on new commits.}