mgmt datadog, add checkstyle suppression

[XiaofeiCao]

Description

Fix datadog migration checkstyle: https://dev.azure.com/azure-sdk/public/_build/results?buildId=5044267&view=logs&j=83516c17-6666-5250-abde-63983ce72a49&t=00be4b52-4a63-5865-8e02-c61723ad0692&l=751

Due to codegen update, output-only models are annotated with @Immutable now.

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• The pull request does not introduce [breaking changes]
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[Copilot]

Pull Request Overview

This PR adds a Checkstyle suppression for the DatadogSetPasswordLinkInner model, which is now annotated @Immutable but has a generated setter method that triggers the ImmutableClassCheck.

• Added suppression entry in checkstyle-suppressions.xml for the Datadog model.
• Included a descriptive comment explaining the suppression.

Comments suppressed due to low confidence (1)

eng/code-quality-reports/src/main/resources/checkstyle/checkstyle-suppressions.xml:118

• [nitpick] Consider clarifying this comment to explain why the suppression is needed, for example: // Suppress ImmutableClassCheck for DatadogSetPasswordLinkInner (output-only model with a generated setter).

  <!-- Suppress the @Immutable class with a property named "setPasswordLink"  -->