feat: support optional unresolved request fields

[moonlander101]

Addresses #508

Alongside plugin changes

Changes:

• Adds a Required checkbox to request-table rows alongside Enabled.
• Omits enabled-but-optional fields when their variables cannot be resolved.
• Preserves validation errors for enabled and required fields with unresolved variables.
• Keeps existing rows required by default for backward compatibility.
• Adds aligned headers for Enabled, Required, Key, Value, and Description.
• Passes required-field metadata through request construction and secure execution.
• Adds tests for validation, omission behavior, hybrid requests, and header alignment.
• Validation: 12 focused tests and the executor TypeScript check passed.