Bump hardhat to version 2.28.0 and update related dependencies

[YoshihitoAso]

📌 Description

This pull request primarily updates the project version from 25.9 to 25.12 across all relevant files and makes minor dependency and metadata adjustments. There are no functional or logic changes to the codebase.

✅ Related Issues

None

🔄 Changes

Version and metadata updates:

• Updated the project version to 25.12 in README.md, README_JA.md, app/__init__.py, app/main.py, docs/ibet_wallet_api.yaml, and pyproject.toml to reflect the new release. [1] [2] [3] [4] [5] [6]
• Updated the hardhat development dependency version in package.json from ~2.26.3 to ~2.28.0.
• Refactored the development dependencies section in pyproject.toml to use [dependency-groups] instead of [tool.uv].

Code cleanup:

• Removed unused imports from app/model/mail/token_provider.py for better code hygiene.

📌 Checklist

• I have added tests where necessary.
• I have updated the documentation where necessary.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
app
__init__.py	1	0	100%
main.py	140	19	86%	103–104, 108, 113–115, 144–145, 187, 197–199, 208–209, 212, 216, 221, 225, 347
app/model/mail
token_provider.py	41	1	97%	41
TOTAL	45776	1584	96%

Tests	Skipped	Failures	Errors	Time
1538	0 💤	0 ❌	0 🔥	12m 24s ⏱️