Add ResultExt context builder and bump version

[RAprogramm]

Summary

• introduce the app_error::Context builder for configuring codes, metadata, caller tracking, and redaction flags when promoting errors
• add the ResultExt::ctx helper with tests to wrap Result errors into masterror::Error, and re-export the new API surface
• bump the crate to 0.12.1, updating changelog and README versions alongside new rustdoc examples

Testing

• cargo +nightly fmt --
• cargo +1.90.0 clippy -- -D warnings
• cargo +1.90.0 build --all-targets
• cargo +1.90.0 test --all
• cargo +1.90.0 doc --no-deps
• cargo audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d1f743d8a8832b99ec94c65b03a12d