Implement WireframeError error integration (#513)

[leynos]

Summary

This branch implements standard error integration for WireframeError in support of issue #513. It makes the default public error type usable through std::error::Error, preserves source chaining for transport and codec failures, and lets TestError use thiserror's transparent #[from] conversion.

Closes #513.

Review walkthrough

• Start with src/error.rs to review the std::error::Error implementation and source chaining behaviour.
• Then review src/testkit/result.rs for the transparent TestError::Wireframe conversion.
• Finish with tests/error_display.rs for the coverage around WireframeError<()> sources.

Validation

• make check-fmt: passed
• make lint: passed
• make test: passed
• coderabbit review --agent: passed with zero findings after one rate-limit retry

Notes

The originally suggested blanket Display/Error implementation plus a separate WireframeError<()> specialisation conflicts under Rust's coherence rules because upstream crates may add future trait implementations for (). This branch therefore uses a stable-compatible Error implementation for WireframeError<E> where E: Debug + 'static; source() exposes the nested Io and Codec errors that are always known to implement Error.

Summary by Sourcery

Integrate WireframeError with the standard error trait and tighten how error sources are exposed.

New Features:

• Allow WireframeError to be used as a std::error::Error without requiring the inner error type to implement Error.
• Support transparent conversion from WireframeError into TestError via thiserror's #[from] attribute.

Enhancements:

• Restrict WireframeError::source to expose only I/O and codec errors as underlying sources, treating protocol and duplicate-route variants as source-less.
• Extend tests to cover WireframeError<()> Error behavior and refine expectations around which variants expose error sources.

Tests:

• Add coverage to validate std::error::Error implementation for WireframeError<(),> including source preservation for I/O and codec errors and absence of sources for protocol and duplicate-route variants.

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Reviewer's Guide

Integrates WireframeError with std::error::Error in a coherence-safe way, exposes only IO and codec errors as sources, and simplifies TestError by using thiserror’s transparent #[from] conversion, with tests updated to cover the new Error behavior for WireframeError and WireframeError<()> specifically.

File-Level Changes

Change	Details	Files
Adjust WireframeError’s std::error::Error implementation to relax trait bounds and refine which variants expose error sources.	Change Error impl bound on WireframeError from requiring E: Debug + Error + 'static to only E: Debug + 'static for stability and coherence with potential upstream implementations. Update source() to return Some(...) only for Io and Codec variants while returning None for Protocol and DuplicateRoute, ensuring consistent and specialization-free source exposure.	src/error.rs
Leverage thiserror’s transparent conversion for TestError::Wireframe and remove the manual From implementation.	Change TestError::Wireframe variant to #[error(transparent)] and accept #[from] WireframeError so TestError can be constructed from WireframeError via derive-generated conversion. Remove the now-redundant manual impl From for TestError. Keep existing conversions like From<&str> and mpsc::SendError unchanged.	src/testkit/result.rs
Update and extend tests to reflect new WireframeError Error behavior and to validate Error implementation for WireframeError<()> specifically.	Modify wireframe_error_messages test so Protocol variants are expected not to expose a source, aligning with the new Error implementation. Add wireframe_error_unit_implements_error test to assert that WireframeError<()> implements Error, preserves sources for Io and Codec variants via downcasting, and does not expose sources for Protocol or DuplicateRoute. Retain and rely on existing tests like wireframe_error_exposes_sources_for_io_and_codec to ensure Io and Codec source chaining behavior remains correct.	tests/error_display.rs

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#513	Ensure WireframeError implements Display with meaningful messages for all variants so it can be converted to user-facing strings.	✅
#513	Ensure WireframeError implements std::error::Error (including for the default WireframeError<()>) so it participates in the standard error ecosystem and exposes appropriate sources for underlying IO/codec errors.	✅
#513	Update TestError to use thiserror’s transparent #[from] conversion for WireframeError instead of a manual From impl.	✅

Possibly linked issues

• WireframeError<E> should derive Display and std::error::Error #513: PR fulfills the issue by adding Error impl for WireframeError and switching TestError::Wireframe to #[from]/transparent.

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