Harden radio save_other_choice lookup for JSON fields

[meravi]

Summary

This hardens acf_field_radio::update_value() when save_other_choice is enabled and the runtime field is JSON-only or missing identifiers.

Changes

• Guard selector resolution (ID -> key -> bail early).
• Bail if acf_get_field() fails or returns a field without DB ID.
• Normalize choices to an array before appending custom values.
• Add regression tests for JSON-only/missing-identifier edge cases.

Tests

• Added:
  • test_update_value_save_other_choice_handles_json_field_without_id
  • test_update_value_save_other_choice_handles_missing_field_identifier

Fixes #381

[github-actions]

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Props ravikhadka.

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