test: Add DecodeError stack test cases

[oab24413gmai]

Summary:

• Add unit tests for DecodeError construction and Display formatting: cover the nested-message path stack (push/truncate behavior via decode_context-style usage) and verify that error messages render the field path correctly for both empty and deeply-nested cases, plus a From for std::io::Error round-trip assertion. Pure additive tests in the existing #[cfg(test)] module, no behavior change.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[caspermeijn]

Thank you for your contribution

[caspermeijn]

I fixed main. Could you rebase the PR?

[caspermeijn]

@oab24413gmai Please rebase the branch