[Good First Issue]: Implement `XmlError` → `std::error::Error` trait implementation (if missing)

[AteebNoOne]

Description

For idiomatic interop with the wider Rust ecosystem (e.g., anyhow, ? operator across crate boundaries), XmlError should implement std::error::Error (behind the std feature) and core::fmt::Display unconditionally. Verify current state and fill any gaps.

Acceptance Criteria

• XmlError implements Display in both std and no_std builds
• XmlError implements std::error::Error when the std feature is enabled
• A unit test confirms XmlError can be used with Box<dyn std::error::Error>
• cargo clippy and cargo test pass with no new warnings

Helpful Resources & Context

• Core error module in crates/tinyxml2/src/
• Rust API guidelines on error types: https://rust-lang.github.io/api-guidelines/interoperability.html#error-types-are-meaningful-and-well-behaved-c-good-err

Implementation Notes

Gate the std::error::Error impl behind #[cfg(feature = "std")] to preserve no_std compatibility from Phase 9.

[anidroid1184]

Hi @AteebNoOne , can you asing me this issue?

[anidroid1184]

Hi @AteebNoOne i'll been working on all issues today