Warn when launch timeout is set below the recommended minimum

[om7057]

Fixes #899

SSHConnector.doCheckLaunchTimeoutSeconds only rejected negative or non-numeric values. A small but positive value, for example 1 second, passed validation silently even though it is known to make agent connections flaky, since it does not leave enough time for the SSH handshake and the remoting handshake to complete.

This adds a warning, not an error, when the value is positive but below the plugin's own default of 60 seconds (SSHLauncher.DEFAULT_LAUNCH_TIMEOUT_SECONDS). The behavior of existing configurations that intentionally use a low timeout is not changed, and the value itself is not modified, only a warning is shown on the configuration page.

I deliberately did not enforce a hard floor by clamping the value, since SSHLauncherTest.timeoutAndRetrySettings already asserts that an explicit value like 39 seconds is honored exactly, and other tests confirm only non-positive values fall back to the default. Silently overriding a user-supplied value below 60 would have broken that existing, intentional behavior.

The check lives in SSHConnector rather than SSHLauncher. SSHLauncher's own config.jelly only renders the host field directly and delegates rendering of every other field, including launchTimeoutSeconds, to SSHConnector's config.jelly and descriptor via <st:include page="config.jelly" class="${descriptor.sshConnectorClass}"/>. So this single check in SSHConnector.DescriptorImpl already covers both the SSHConnector-based cloud connector UI and the SSHLauncher-based permanent agent UI.

Testing done

Added SSHConnectorTest with 7 cases covering: below minimum (warning), at minimum (ok), above minimum (ok), blank (ok), negative (error), not a number (error), and zero (ok, since zero is handled separately by falling back to the default elsewhere and should not also warn here).

Ran the full existing test suite locally on JDK 25 (matching this repo's linux CI leg): all passing, including SSHLauncherTest.timeoutAndRetrySettings which confirms this change does not alter the existing timeout value behavior. Also verified ./mvnw spotless:check passes clean.

Submitter checklist

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[Copilot]

Pull request overview

This PR improves configuration-time validation for SSH agent launch timeouts by warning (rather than failing) when users enter a positive timeout below the plugin’s recommended minimum (the default 60s), addressing flakiness reported in #899 while preserving existing runtime behavior.

Changes:

• Add a FormValidation.Kind.WARNING when launchTimeoutSeconds is positive but below DEFAULT_LAUNCH_TIMEOUT_SECONDS.
• Add a localized warning message for the new validation outcome.
• Add a focused regression test suite covering warning/ok/error cases for the launch timeout field.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/main/java/hudson/plugins/sshslaves/SSHConnector.java	Adds a warning for low positive launch timeout values during descriptor form validation.
src/main/resources/hudson/plugins/sshslaves/Messages.properties	Adds the new i18n message used for the low-timeout warning.
src/test/java/hudson/plugins/sshslaves/SSHConnectorTest.java	Adds regression tests covering the new warning behavior and existing error/ok cases.

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[om7057]

Applied in 5497fa2, thanks for the catch.