test(alias): cover non-UTF8 RC_HOST credential decoding

[overtrue]

Summary

• Add focused rc-core coverage for RC_HOST credentials that contain valid percent escapes but decode to invalid UTF-8.
• Assert the error is classified as invalid percent-encoding for the secret key and does not expose credential material.

Background

Recent RC_HOST credential parsing changes added explicit malformed percent-encoding handling. The non-UTF-8 decode error branch remained uncovered, so this PR adds a narrow regression test around that path.

Validation

• cargo test -p rc-core test_parse_rc_host_alias_rejects_non_utf8_percent_encoded_secret_key --lib
• cargo test -p rc-core alias::tests --lib
• make pre-commit

[Copilot]

Pull request overview

Adds a targeted regression test in rc-core to cover the previously-uncovered error branch where RC_HOST credentials contain syntactically valid percent-escapes that decode into invalid UTF-8 (e.g., %FF). The test asserts the error is reported as invalid percent-encoding for the secret key and that the error message does not leak credential material.

Changes:

• Add a unit test exercising non-UTF8 percent-decoded secret key handling in parse_env_alias.
• Assert the returned error message is appropriately classified and does not include access/secret credential substrings.

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