[Bug] TransactionRequest conversions erase contract creation and can_build disagrees with build_unsigned

### Component

network, json-rpc, rpc

### What version of Alloy are you on?

Alloy v2.1.1 (bce5f50d63bb7d6d9a3b5b455a9a4508bf9dcdc4)

### Operating System

Windows

### Describe the bug

`TransactionRequest` conversions from concrete transaction types erase `TxKind::Create`.

For a valid legacy deployment, `TransactionRequest::from_transaction(deployment)` preserves `Some(TxKind::Create)` and rebuilds the original transaction. In contrast, both `TypedTransaction::Legacy(deployment).into()` and signed `TxEnvelope::Legacy(...).into()` produce `to == None`.

For each lossy request:

- `can_build() == true`;
- `complete_preferred() == Err(vec!["to"])`;
- `build_unsigned()` returns an error.

`Some(TxKind::Create)` is the public representation needed to distinguish deployments from incomplete requests with `to == None`: https://github.com/alloy-rs/alloy/pull/3704

Expected: concrete conversion paths preserve `Some(TxKind::Create)`; readiness and build APIs agree.

Actual: valid deployments become incomplete requests which report themselves buildable and then fail to build.

Trace TransactionRequest::from_transaction, TypedTransaction::Legacy(...).into(), and signed TxEnvelope::Legacy(...).into() first, then compare their handling of deployment transactions. Verify the affected requests preserve Some(TxKind::Create), while can_build(), complete_preferred(), and build_unsigned() consistently agree that the request is ready.

Rédigé par le modèle d'indexation à partir du texte de l'issue.