Add configurable SSH port for agent connections

[ToshipSo]

Summary

• Adds a configurable SSH port field to Instance Configuration (defaults to 22)
• Removes the hardcoded SSH_PORT = 22 constant from ComputeEngineComputerLauncher
• Includes UI field, form validation (1-65535), inline help, and backward compatibility for existing serialized configs

Resolves #134

Changes

• InstanceConfiguration - new sshPort field with @DataBoundSetter, descriptor validation (1-65535), builder support
• ComputeEngineInstance - stores sshPort with backward-compatible getter for older serialized configs
• ComputeEngineComputerLauncher - uses node.getSshPort() instead of hardcoded SSH_PORT constant
• config.jelly - SSH Port number field in Launch Configuration section
• help-sshPort.html - inline help

Testing done

Automated Tests

Six tests in InstanceConfigurationTest cover the new SSH port logic:

Test	What it verifies
testSshPortDefaultValue	setSshPort(null) falls back to default port 22
testSshPortCustomValue	setSshPort(2222) stores and returns 2222
testSshPortInvalidValuesFallBackToDefault	Out-of-range values (0, -1, 70000) all fall back to 22
testSshPortBoundaryValues	Boundary ports 1 and 65535 are accepted
descriptorSshPortValidation	Form validation accepts valid ports (1, 22, 2222, 65535), rejects invalid input (0, -1, 65536, "abc"), and permits empty/null
testConfigRoundtrip	Updated to include sshPort — verifies the field survives a Jenkins save/load cycle through the UI form
All tests pass via mvn test -Dtest=InstanceConfigurationTest.

Manual Testing

Launched Jenkins locally with mvn hpi:run and verified:

1. UI field present: The "SSH Port" field appears in the Launch Configuration section with a default value of 22
2. Inline help: The (?) icon next to the field displays the help text
3. Validation feedback: Entering invalid values (0, 70000, "abc") shows an error; valid ports (22, 2222, 443) show no error

4. Persistence: Set port to 2222, saved config, reopened — value persisted correctly

5. Default behavior: Left the field at 22 (default), saved — behaves identically to previous plugin behavior

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed