[BUG] Txn iterator constructors panic on caller misuse

[feichai0017]

🐛 Bug Description

Txn.NewIterator / Txn.NewKeyIterator panic on invalid runtime state and invalid options instead of returning a normal error.

Affected code:

• txn_iterator.go:113
• txn_iterator.go:170

Current panic cases include:

• transaction already discarded
• DB already closed
• NewKeyIterator called with non-empty opt.Prefix

🛠 Reproduction Steps

Steps to reproduce the behavior:

1. Create a transaction and call Discard().
2. Call txn.NewIterator(...) or txn.NewKeyIterator(...).
3. Or call txn.NewKeyIterator(key, IteratorOptions{Prefix: []byte("x")}).
4. Observe process panic.

📋 Expected Behavior

Iterator construction should fail with an explicit error path (or safe API variant) instead of panicking and potentially crashing the service process.

📸 Screenshots / Logs

Observed panic messages:

• Transaction has already been discarded
• opt.Prefix should be nil for NewKeyIterator.

💻 Environment

• OS: macOS (arm64)
• Go Version: 1.26
• NoKV Version: main (2026-02-14)

🧐 Additional Context

This is an API robustness issue. panic may be acceptable for internal invariants, but these paths are easy to trigger from caller misuse and can crash long-running services.