fix: fix secret handling without curly braces

[aaltat]

Fixes #4896

[Copilot]

Pull request overview

Fixes Robot Framework Secret handling for Type Secret/Fill Secret when secrets are passed using $var syntax (without curly braces), preventing a TypeError during exception masking and ensuring the original selector-related error surfaces correctly (Fixes #4896).

Changes:

• Unwrap resolved Secret values to their underlying .value string when resolving $var/%var placeholders.
• Extend acceptance tests to cover failing Type Secret/Fill Secret cases for both ${secret} and $secret syntaxes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Browser/base/librarycomponent.py	Ensures placeholder resolution returns a plain string when the resolved value is a Secret, avoiding str.replace(..., Secret) failures.
atest/test/02_Content_Keywords/secret.robot	Adds regression coverage for failure paths of Type Secret/Fill Secret with and without curly-brace variable syntax.