fix: add __call__ to MockOptional for WTForms validator compatibility

[YoannAbriel]

HookMetaService.MockOptional lacks a __call__ method. When FAB initializes WTForms forms during POST /auth/token, stored MockOptional instances fail check_validators because they're not callable — every token request returns 500.

Added __call__(self, form, field) as a no-op so MockOptional satisfies the WTForms validator protocol. Added unit tests.

Closes: #63803

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4, claude-opus-4-6)

Generated-by: Claude Code (Opus 4, claude-opus-4-6) following the guidelines

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