#3586428 Fix TextExtractor to include all FieldConfig fields regardless of name prefix

Description

Replaces the hardcoded field-name pattern check in TextExtractor::shouldExtract() with an instanceof FieldConfigInterface check. Previously, only fields matching body, info, ^field_, or ^layout_builder_ were extracted for translation — silently skipping any custom field whose name does not follow the field_ convention. Since the field_ prefix is configurable (and can be emptied) via field_ui.settings.yml, this affected entire projects.

All FieldConfigInterface fields (custom configurable fields, regardless of name) now defer to the plugin's own shouldExtract() logic. Base fields continue through the existing label-key check.

Testing instructions

1. Set field_prefix to empty in field_ui.settings.yml.
2. Create a plain text field named summary on an Article node.
3. Populate the field and run AI translation on the node.
4. Verify the summary field content is translated.

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

AI Compliance

Note

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Closes #3586428