Add MyNumberValue type: enforce My Number format, API improvements, tests

[hsaito]

Add MyNumberValue type: enforce My Number format, API improvements, tests

Summary

This pull request introduces MyNumberValue, an immutable value object that validates and enforces the correct My Number format (including the check digit) at construction time. It also updates the API controller to provide endpoints that accept and return MyNumberValue, adds comprehensive unit tests, and updates the README with usage examples.

Changes

• Add MyNumberValue struct to MyNumberNET (validation, parsing, formatting, conversions).
• Update MyNumberController to provide endpoints that create/format/verify MyNumberValue.
• Add unit tests MyNumberValueTests and update controller tests.
• Update README.md with MyNumberValue usage examples.

Motivation

Using a strongly-typed, validated value object prevents invalid My Number data from propagating through the system and centralizes validation and formatting logic.

Testing

• All unit tests pass locally: dotnet test (38 passing tests).
• dotnet build succeeds locally.

Checklist

• New tests added for MyNumberValue and controller changes.
• README updated with examples.
• Build passes locally.

Notes for reviewers

• Focus on MyNumberNET/MyNumber.cs (new MyNumberValue struct) and MyNumberNET_ApiServer/Controllers/MyNumberController.cs.
• Pay attention to ToString format options and conversion operators. Suggest changes if another representation or API surface is preferred.
• If preferred, I can split this into smaller PRs (e.g., docs / tests / implementation).

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