[ruff] Stabilize class-with-mixed-type-vars (RUF053) (#18512)

This PR stabilizes the RUF053 rule by moving it from preview to stable
status for the 0.12.0 release.

## Summary
- **Rule**: RUF053 (`class-with-mixed-type-vars`)
- **Purpose**: Detects classes that have both PEP 695 type parameter
lists while also inheriting from `typing.Generic`
- **Change**: Move from `RuleGroup::Preview` to `RuleGroup::Stable` in
`codes.rs` and migrate preview tests to stable tests

## Verification Links
- **Tests**:
[ruff/mod.rs](https://github.com/astral-sh/ruff/blob/main/crates/ruff_linter/src/rules/ruff/mod.rs#L98)
- Shows RUF053 moved from preview_rules to main rules test function
- **Documentation**:
https://docs.astral.sh/ruff/rules/class-with-mixed-type-vars/ - Current
documentation shows preview status that will be automatically updated

crates/ruff_linter/src/codes.rs

@@ -1019,7 +1019,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "049") => (RuleGroup::Preview, rules::ruff::rules::DataclassEnum),
         (Ruff, "051") => (RuleGroup::Stable, rules::ruff::rules::IfKeyInDictDel),
         (Ruff, "052") => (RuleGroup::Preview, rules::ruff::rules::UsedDummyVariable),
-        (Ruff, "053") => (RuleGroup::Preview, rules::ruff::rules::ClassWithMixedTypeVars),
+        (Ruff, "053") => (RuleGroup::Stable, rules::ruff::rules::ClassWithMixedTypeVars),
         (Ruff, "054") => (RuleGroup::Preview, rules::ruff::rules::IndentedFormFeed),
         (Ruff, "055") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryRegularExpression),
         (Ruff, "056") => (RuleGroup::Preview, rules::ruff::rules::FalsyDictGetFallback),