fix: support $guarded models and MariaDB driver [3.x]

[ManukMinasyan]

Summary

• Models using $guarded instead of $fillable couldn't save custom fields. Replace mergeFillable with isFillable() and fillableFromArray() overrides.
• Add MariaDB to text_value driver-specific constraints (matches MySQL at 65535).

Closes #121
Closes #120

Test plan

• Guarded model recognizes custom_fields as fillable via isFillable() override
• Guarded model can mass-assign custom_fields
• Guarded model stores custom_fields in temp storage when filling
• MariaDB included in text_value constraints
• MariaDB constraint matches MySQL value
• Full test suite passes (no regressions)

[Copilot]

Pull request overview

This PR addresses two compatibility gaps in the custom-fields package: (1) enabling mass-assignment of custom_fields for models that rely on $guarded instead of $fillable, and (2) explicitly supporting Laravel’s mariadb database driver in driver-specific constraints.

Changes:

• Update UsesCustomFields to add custom_fields to fillable when $guarded is non-empty.
• Add mariadb to the text_value driver-specific maximum constraints (matching MySQL at 65535).
• Add Pest tests covering MariaDB constraints and guarded-model fill behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/Models/Concerns/UsesCustomFields.php	Adjusts when custom_fields is merged into $fillable to support guarded models.
src/Support/DatabaseFieldConstraints.php	Adds explicit mariadb max length for text_value.
tests/Feature/Models/GuardedModelCustomFieldsTest.php	Adds tests for guarded-model custom_fields fillability.
tests/Feature/Support/DatabaseFieldConstraintsTest.php	Adds tests verifying MariaDB is present and has the expected max.