Optimize context metadata redaction handling

[RAprogramm]

Summary

• avoid reapplying field redaction policies when context metadata already reflects them while preserving the fallback for contexts without fields
• add a unit test to cover policies applied before metadata insertion

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_68d781a1f09c832b81a49c6fa8991158