fix(forms): preserve custom-control focus context in signal forms

[cexbrayat]

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
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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?

Without this, custom focus methods that access instance members throw at runtime when focusBoundControl() is called.

Issue Number: #67051

What is the new behavior

Store custom control focus callbacks in a wrapper so method invocation keeps the original object context.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[JeanMeche]

Thx for this !

[JeanMeche]

(signals forms have diverged between patch & minor, so we'll have to target minor for this).

[mattrbeck]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 63d8005