Return error when fetching ledger entry for trustline using native asset.

[fnando]

What

Return error when fetching ledger entry for trustline using native asset.

Why

So it behaves like it should with native asset. Close #2394.

Known limitations

N/A

[Copilot]

Pull request overview

This PR addresses issue #2394 by returning an error when users attempt to fetch a trustline ledger entry using the native asset (XLM). Previously, the CLI would silently accept "XLM" or "native" as the asset parameter and return an empty result, which was confusing since trustlines cannot hold the native asset - native balances are stored in the account entry itself.

Changes:

• Added a new NativeAsset error variant that provides a helpful error message directing users to use stellar ledger entry fetch account instead
• Modified the asset parsing logic to check for "XLM" or "native" (case-insensitive) and return the new error instead of creating a TrustLineAsset::Native
• Added comprehensive test coverage for both "xlm" and "native" keywords with various capitalizations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
cmd/soroban-cli/src/commands/ledger/entry/fetch/trustline.rs	Added NativeAsset error variant and modified asset parsing to reject native assets with a helpful error message
cmd/crates/soroban-test/tests/it/integration/ledger/entry.rs	Added two test functions covering all case variations of "xlm" and "native" to ensure proper error handling