Tracker: task type custom attributes not shown in "New issue" dialog, and "Required" not enforced #318

Follow-up to #94 / hcengineering/platform#9071.

#94 fixes task type attributes in the issue side panel, but two related gaps remain in the Tracker:

1. The "New issue" dialog does not render the custom attributes of the selected task type, so they can't be filled in at creation.
2. Attributes marked as "Required" are not enforced: the issue can be created (and the value cleared later) without filling them.

Steps to reproduce

1. Settings > Space types > [project type] > [task type] > Properties: add a custom attribute (Text) and enable "Required".
2. In the Tracker, open "New issue" and select that task type.
3. Fill in only the title and click "Create issue".

Actual

• The custom attribute is not shown in the dialog.
• The issue is created without it (Required is not enforced).

Expected
When creating an issue, the dialog shows the task type's custom attributes and blocks "Create issue" until required ones are filled (similar to "Required attributes" for Cards, hcengineering/platform#10918).

Environment
Version 0.7.426 (self-hosted, MODEL_VERSION 0.7.426). This version doesn't include #94, but based on the files changed in #94 (issue side panel only), the "New issue" dialog and Required enforcement are not covered by that fix.