fix: avoid mutating strings when building results

[G-Rath]

The current use of prepend results in deprecation warnings on Ruby 3.4 since in the future strings will be frozen by default so this changes the implementation to build a new string instead

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[Copilot]

Pull Request Overview

This PR addresses Ruby 3.4 deprecation warnings by eliminating string mutation operations. The changes replace prepend method calls with string interpolation to build new strings instead of modifying existing ones, preparing the codebase for Ruby's future default string freezing behavior.

Key Changes:

• Replaced mutating prepend calls with non-mutating string interpolation in two locations

Reviewed Changes

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

File	Description
packages/axe-core-api/lib/axe/api/results/rule.rb	Updated indent lambda to use string interpolation instead of prepend for adding leading spaces
packages/axe-core-api/lib/axe/api/results/checked_node.rb	Updated failure message formatting to use string interpolation instead of prepend for adding bullet points

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[scottmries]

Thanks for this! Closing it for #462, which will allow our tests to run.