Check if YAML extension is installed before registering config schema

[SparshGarg999]

Description

This PR resolves issue #12948 where extension activation fails when the Red Hat YAML extension (redhat.vscode-yaml) is not installed.

Writing to yaml.schemas user settings when the configuration is not registered throws CodeExpectedError: Unable to write to User Settings because yaml.schemas is not a registered configuration..

This is resolved by checking if the redhat.vscode-yaml extension is installed using vscode.extensions.getExtension before reading or updating the yaml.schemas settings configuration.

Test Plan

1. Uninstall the Red Hat YAML extension.
2. Start VS Code with the Continue extension enabled.
3. Verify that the extension activates successfully without throwing a CodeExpectedError on yaml.schemas.

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[SparshGarg999]

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