fix(query-core): ensure onMutate runs synchronously when cache config is undefined (#8724)

[xxziiko]

🎯 Changes

Previously, mutationCache.config.onMutate was always awaited using optional chaining (await this.#mutationCache.config.onMutate?.()), which caused unnecessary async behavior even when the callback was undefined.

• Changed to conditional check: only await when onMutate is actually defined
• Added regression test to verify synchronous execution when cache config is not provided

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).