Fix #5205: Handle invalid SubscriptionMode values safely

[nagarjunahp88-byte]

This PR fixes issue #5205 by improving SubscriptionModeConverter.

Changes:

• Safely handles null and empty values
• Normalizes input using trim and uppercase
• Prevents IllegalArgumentException for invalid enum values

This ensures the application does not crash on invalid input.

[github-actions]

Welcome to the Apache EventMesh community!!
This is your first PR in our project. We're very excited to have you onboard contributing. Your contributions are greatly appreciated!

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[Pil0tXia]

plz check code style

[nagarjunahp88-byte]

Thanks for the review!
I’ve updated the code style and added unit tests to cover invalid and null SubscriptionMode values.
Please take another look.

[nagarjunahp88-byte]

Hi, I’m a new contributor.
I tried joining the ASF Slack via https://s.apache.org/slack-invite
but I’m unable to join since I don’t have an @apache.org email yet.
Could someone please help me with Slack access or guide me to the right channel?
Thanks!

[nagarjunahp88-byte]

The CI failure seems to be related to Gradle build scan publishing
(develocity.apache.org authentication required), not a compilation
or test failure.

Please let me know if this is expected or if any change is needed
from my side.