Add test assertion for install_extension_in_lib default value

[Copilot]

What was the end-user or developer problem that led to this PR?

The test_initialize method in test_gem_config_file.rb did not explicitly verify the default value of install_extension_in_lib when no configuration is provided. This left a gap in test coverage for the new DEFAULT_INSTALL_EXTENSION_IN_LIB = false constant introduced in PR #9240.

What is your fix for the problem, implemented in this PR?

Added assert_equal false, @cfg.install_extension_in_lib in the first section of test_initialize (line 46), which tests default values before any config file overrides are applied. This mirrors the existing assertions for other default config values like ipv4_fallback_enabled.

The assertion validates that the config object correctly initializes with install_extension_in_lib set to false when no .gemrc configuration specifies otherwise.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[Copilot]

Pull request overview

This PR adds a test assertion to verify that the install_extension_in_lib configuration property defaults to false when no configuration file specifies a value. This improves test coverage for the DEFAULT_INSTALL_EXTENSION_IN_LIB constant that was introduced in PR #9240.

• Added assertion to verify default value of install_extension_in_lib is false

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