feat(models): type ledger_entry response node using LedgerEntry enum

[soloking1412]

High Level Overview of Change

Types the ledger_entry result's node field using the existing LedgerEntry enum from models::ledger::objects, replacing the untyped Option<serde_json::Value>.

// before
pub node: Option<serde_json::Value>,
// after
pub node: Option<crate::models::ledger::objects::LedgerEntry<'a>>,

Since the models::ledger::objects::LedgerEntry enum already existed (used by Ledger::account_state for full-ledger state dumps), this reuses it rather than introducing a second, parallel discriminated-union type — one enum, two call sites.

Blockers noted in #308

The issue calls out that the real obstacle is XRPL's wire format: a flat object carrying its own LedgerEntryType discriminator ({"LedgerEntryType": "AccountRoot", "Account": ..., ...}), which doesn't fit serde's default externally-tagged enum representation, and doesn't fit #[serde(tag = "LedgerEntryType")] either without removing ledger_entry_type from CommonFields (a breaking change across every ledger object struct). This implements the issue's suggested workaround: a hand-written Deserialize for LedgerEntry that buffers to serde_json::Value, reads LedgerEntryType to pick the variant, then delegates to serde_json::from_value for that variant's own Deserialize. Serialize is likewise hand-written to flatten straight through to the inner type instead of wrapping it — the enum's default derive was serializing as {"AccountRoot": {...}}, which doesn't match real rippled output either, and was already silently wrong for Ledger::account_state before this change (nothing previously exercised that path with real wire-format JSON).

Unknown/unrecognized LedgerEntryType values are a deserialize error (unknown_variant) rather than silently dropped, so a future ledger object type rippled adds before this crate models it fails loudly instead of losing data.

Context of Change

Filed as #308. Overlaps in idea with the node typing included in #160 (open since April, part of a much larger integration-tests PR) — this is a standalone implementation of just that piece, reusing the existing LedgerEntry enum instead of #160's parallel LedgerEntryNode type, so it doesn't carry a duplicate 26-variant definition and doesn't need an Unknown(Value) catch-all. Flagging the overlap here so it's easy to reconcile with #160 either way.

Type of Change

• New feature (non-breaking change which adds functionality)
• Tests

Before / After

Deserializing a non-AccountRoot entry — the exact scenario #308 says used to fail before node was widened past a hardcoded AccountRoot struct — now dispatches correctly:

let entry: LedgerEntry = serde_json::from_str(r#"{
    "LedgerEntryType": "DirectoryNode",
    "Owner": "rN7n...",
    ...
}"#)?;
match entry.node.unwrap() {
    LedgerEntry::DirectoryNode(directory_node) => { /* typed access */ }
    _ => unreachable!(),
}

Ledger::account_state (the other consumer of models::ledger::objects::LedgerEntry) now round-trips against the real flat wire format too, as a side effect of fixing Serialize/Deserialize on the shared enum.

Test Plan

• cargo test --release: 1316 passed (up from 1313 on main).
• Added to models::ledger::objects: a flat-wire-format deserialize test for a non-AccountRoot entry (DirectoryNode), an unknown-LedgerEntryType rejection test, and a missing-LedgerEntryType rejection test. Tightened the existing round-trip test to assert the wire format is no longer externally-tagged.
• Rewrote models::results::ledger_entry's existing tests (deserialize, round-trip, DirectoryNode) to pattern-match the typed enum instead of indexing into serde_json::Value.
• Updated the two integration tests in tests/requests/ledger_entry.rs that inspected node as raw JSON (credential, vault_by_id) to match on the typed variant instead — compiled clean under cargo check --tests --features std,json-rpc,helpers,cli,websocket,integration (these need a live xrpld node to actually execute, so compiled but not run end-to-end here).
• cargo clippy --all-targets --features std,json-rpc,helpers,cli,websocket,integration and cargo fmt --check: clean on all changed files.

Closes #308