tests: Add integration tests for RPC requests

[pdp2121]

High Level Overview of Change

• Resolves Add integration tests for RPCs #146
• Removes some AI-generated reference to xrpl.js
• Replaces the opaque Option<serde_json::Value> node field in the LedgerEntry result with a typed LedgerEntryNode enum — a discriminated union over all ~20 ledger object types (like AccountRoot, DirectoryNode, Offer, etc.) — matching how xrpl.js uses a LedgerEntry union type. To work around serde's limitation where #[serde(tag = "...")] conflicts with #[serde(flatten)] in the inner structs, implements a custom Deserialize that buffers the JSON, peeks at the LedgerEntryType field to dispatch to the correct variant, and falls back to Unknown(Value) for unrecognized types. This allows type-safe pattern matching on ledger objects while remaining resilient to new ledger entry types added to rippled.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Tests (You added tests for code that already exists, or your new feature included in this PR)
• Documentation Updates
• Release

[pdp2121]

/ai-review