∴ Vector: Centralize display name validation using Pydantic Annotated

[ToolchainLab]

• 💡 What: Centralized duplicated display_name validation logic into a reusable Pydantic V2 type (DisplayNameStr using Annotated and BeforeValidator). Replaced repeated @field_validator methods in RegisterRequest and PasskeyRegisterStartRequest. Removed the unused _validate_display_name function.
• 🎯 Why: To reduce duplication, establish a single source of truth for display name normalization/validation, and prevent future drift between models.
• 🧠 Design: Created a pure normalization helper (normalize_display_name) that safely handles types before stripping. Wrapped this in a BeforeValidator within an Annotated type to create a composable validation primitive that perfectly replaces the scattered classmethods.
• λ FP Angle: Transforms scattered, mutation-adjacent class methods into a declarative, type-driven pipeline. Semantics are now locked at the type level rather than repeatedly defined at the field level.
• ✅ Verification: Ran uv run --locked ruff format . && uv run --locked ruff check . && uv run --locked mypy src && uv run pytest ensuring no behavior changes and that standard Pydantic validation errors still surface correctly for empty inputs.
• 🧪 Edge cases: The new normalize_display_name correctly verifies isinstance(v, str) before mutating, allowing non-string inputs to safely fall through to Pydantic's built-in type validation.
• ⚠️ Risk: Extremely low. The behavior is identical to the previous @field_validator implementations, and standard test coverage confirms schemas correctly reject whitespace-only names.

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PR created automatically by Jules for task 9427188932183556861 started by @ToolchainLab

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