Fix UnicodeEncodeError by using UTF-8 encoding for migration files (closes #2096)

[chuwei-dev]

Description

This PR updates migration file writing to explicitly use UTF-8 encoding instead of ASCII.

Previously, migration files were written with:

path.write_text(self.as_string(), encoding="ascii")

This prevented writing non-ASCII characters and could raise:

UnicodeEncodeError: 'ascii' codec can't encode characters

The implementation now uses:

path.write_text(self.as_string(), encoding="utf-8")

---

Motivation and Context

Fixes #2096.

Migration files may contain non-ASCII characters (e.g. verbose names, comments, or field descriptions).
Using a hard-coded ASCII encoding makes such content impossible to write.

UTF-8 is the standard encoding for Python source files and ensures:

• Cross-platform consistency
• Deterministic migration output
• No reliance on system locale defaults

This change removes the unnecessary ASCII restriction and improves robustness.

---

How Has This Been Tested?

Tested locally with:

• Python 3.12
• Windows 10 environment

Steps performed:

1. Created a model containing non-ASCII characters (e.g. Chinese verbose_name).
2. Ran tortoise makemigrations.
3. Confirmed that migration files are generated successfully.
4. Verified that no encoding errors occur.
5. Ensured existing migrations continue to work correctly.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[abondar]

Please fix linter

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

---

_{Comparing chuwei-dev:fix/migrations-loader-default-encoding (c279796) with develop (fe73270)}

[coveralls]

Pull Request Test Coverage Report for Build 21899166276

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.478%

---

Totals
Change from base Build 21856593682:	0.0%
Covered Lines:	10420
Relevant Lines:	12167

---

💛 - Coveralls

[chuwei-dev]

Thanks! Formatting issues have been fixed.