JENKINS-59781 Handle partial SSH output without trailing newlines

[nrayapati]

Adds handling for partial output from standard output and error streams to ensure command output is not truncated when commands exit without trailing newlines. This improves logging completeness for SSH command execution.

Description

See JENKINS-59781.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description.
• Appropriate unit or acceptance tests or explanation to why this change has no tests.
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description.
• Reviewed the code.
• Verified that the appropriate tests have been written or valid explanation given.
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[Copilot]

Pull request overview

This PR addresses JENKINS-59781 by adding support for capturing partial SSH command output that lacks trailing newlines. The change prevents command output from being truncated when SSH commands exit without properly terminated lines.

Key Changes:

• Adds when(partial:) event handlers for both stdout and stderr streams to capture incomplete lines
• Includes trim() and empty checks to avoid logging whitespace-only output

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[long76]

https://issues.jenkins.io/browse/JENKINS-76338