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, cbravobernal.

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[cbravobernal]

Verified locally:

• The two new tests reproduce the reported warning against current trunk (Undefined array key "ID" in class-acf-field-radio.php:347) and pass with this fix.
• Full PHPUnit suite (2230 tests), PHPStan, and phpcs all green on this branch.

The defensive selector build (ID → key → bail) and the acf_get_field() return check both look right, and behavior for valid persisted fields is unchanged.

Verification run via Claude Code, reviewed by @cbravobernal.

[tapheret2]

Review: Harden radio save_other_choice lookup for JSON fields

Files: includes/fields/class-acf-field-radio.php, tests/php/includes/fields/test-class-acf-field-radio.php
Size: +50 / -9

Notes

• Focused diff from a static pass; no obvious red flags, but I only reviewed the patch text.

Nits

• Any user-facing change that should be mentioned in the PR description?
• Need a changelog/release note for the next tag?

Thanks @meravi — independent review pass on the diff.