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This section details on the original issue you should resolve

<issue_title>Replace TSMappedType::type_parameter with dedicated fields to match TSESTree</issue_title>
<issue_description>## Summary

Currently, TSMappedType uses TSTypeParameter to represent its type parameter, which includes several fields that are syntactically invalid in mapped types (r#const, default). We should replace type_parameter with dedicated fields that accurately represent the allowed syntax and match the TSESTree specification.

Current Implementation

pub struct TSMappedType<'a> {
    pub span: Span,
    pub type_parameter: Box<'a, TSTypeParameter<'a>>,
    pub name_type: Option<TSType<'a>>,
    pub type_annotation: Option<TSType<'a>>,
    pub optional: Option<TSMappedTypeModifierOperator>,
    pub readonly: Option<TSMappedTypeModifierOperator>,
    pub scope_id: Cell<Option<ScopeId>>,
}

Problem

1. Semantic Mismatch: TSTypeParameter includes fields that are not valid in mapped type syntax:

   • r#const - mapped types cannot use const modifier
   • default - mapped types cannot have default types
   • in - variance annotation not applicable to mapped types
   • out - variance annotation not applicable to mapped types

2. TSESTree Incompatibility: The current structure doesn't match the TSESTree specification, which defines mapped types with separate key and constraint fields rather than a full type parameter.

This creates both a semantic mismatch where the AST structure allows syntactically invalid constructs and a compatibility issue with TypeScript tooling.

Proposed Solution

Replace type_parameter: Box<'a, TSTypeParameter<'a>> with two explicit fields to match TSESTree:

#[ast(visit)]
#[scope]
#[derive(Debug)]
#[generate_derive(CloneIn, Dummy, TakeIn, GetSpan, GetSpanMut, ContentEq, ESTree)]
pub struct TSMappedType<'a> {
    pub span: Span,
    /// The type parameter name (e.g., `P` in `[P in keyof T]`)
    pub key: BindingIdentifier<'a>,
    /// The constraint (e.g., `keyof T` in `[P in keyof T]`)
    pub constraint: TSType<'a>,
    pub name_type: Option<TSType<'a>>,
    pub type_annotation: Option<TSType<'a>>,
    pub optional: Option<TSMappedTypeModifierOperator>,
    pub readonly: Option<TSMappedTypeModifierOperator>,
    pub scope_id: Cell<Option<ScopeId>>,
}

Note: The existing #[estree(add_fields(key = TSMappedTypeKey, constraint = TSMappedTypeConstraint))] can be removed as these will now be real fields.

Benefits

1. Type Safety: The AST structure will only allow syntactically valid constructs
2. TSESTree Compatibility: Matches the structure expected by TypeScript tooling and TypeScript-ESLint
3. Clarity: The intent and constraints are explicit in the type definition
4. Maintainability: Reduces confusion about which TSTypeParameter fields are applicable
5. Correctness: Prevents the possibility of creating invalid AST nodes

Example

For the mapped type:

type Readonly<T> = { readonly [P in keyof T]: T[P] }

• key would be the identifier P (as BindingIdentifier)
• constraint would be the type keyof T (as TSType)

TSESTree Compatibility

This change aligns Oxc's AST with typescript-eslint's TSESTree specification, which defines TSMappedType with key and constraint fields. This could help us to remove specific converting logic from OXC AST to ESTree</issue_description>

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