fix(forms): resync select when option text changes - #71444

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The MutationObserver that keeps a [formField] bound <select> in sync observes characterData, because an <option> without a value uses its text as its value. isRelevantSelectMutation only returns true when nodes were added or removed, which is never the case for a characterData record. So when an option's text changes to match the model value, the select is not resynced and shows no selection.

Issue Number: N/A

What is the new behavior?

A characterData mutation on a non comment node is treated as relevant, so the select is resynced to the model value. Added a test next to the existing "should resync the select value when an option is added" test.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Fixes #71443