fix(parser): allow in in arrow block bodies within for initializers (#26395)

## Summary

Allow the `in` operator inside an arrow function’s block body when the arrow appears in a `for` initializer:

```js
for (() => { a in b; };;);
```

This is valid JavaScript, but the parser previously reported a missing semicolon at `in`. The block now parses successfully, including async arrows and arrows with a single unparenthesized parameter.

## Cause and fix

A `for` initializer disables `Context::In` so that an unparenthesized `in` can delimit a `for…in` statement. Arrow block bodies inherited that disabled context through `parse_arrow_function_expression_body`, which called `parse_function_body` directly.

The block-body branch now calls `parse_function_body` through `context_add(Context::In, ...)`. This enables `in` throughout the function body and restores the enclosing context afterward. It also permits nested concise arrows inside that block to use `in`.

The change is confined to block bodies because concise expression bodies inherit the surrounding grammar’s `In` parameter. For example, this remains invalid:

```js
for (var f = () => a in b;;);
```

Restoring the enclosing context also keeps this invalid, rather than accidentally treating `a in b` as a regular initializer expression after parsing the first arrow:

```js
for (var f = () => {}, value = a in b;;);
```

crates/oxc_parser/src/js/arrow.rs

@@ -313,7 +313,9 @@ impl<'a, C: Config> ParserImpl<'a, C> {
         self.ctx = self.ctx.and_await(r#async).and_yield(false);
 
         let body = if self.at(Kind::LCurly) {
-            ArrowFunctionBody::FunctionBody(self.parse_function_body())
+            ArrowFunctionBody::FunctionBody(
+                self.context_add(Context::In, Self::parse_function_body),
+            )
         } else {
             // Remove TopLevel context for arrow function expression body
             let expr = self.context_remove(Context::TopLevel, |p| {

tasks/coverage/misc/fail/arrow-block-body-in-context.js

@@ -0,0 +1 @@
+for (var f = () => {}, value = a in b;;);

tasks/coverage/misc/fail/arrow-expression-body-in.js

@@ -0,0 +1 @@
+for (var f = () => a in b;;);

tasks/coverage/misc/pass/arrow-block-body-in.js

@@ -0,0 +1,5 @@
+for (() => { a in b; };;);
+for (async () => { a in b; };;);
+for (x => { return x in b; };;);
+for (async x => { return x in b; };;);
+for (() => { () => a in b; };;);

tasks/coverage/snapshots/codegen_misc.snap

@@ -1,3 +1,3 @@
 codegen_misc Summary:
-AST Parsed     : 85/85 (100.00%)
-Positive Passed: 85/85 (100.00%)
+AST Parsed     : 86/86 (100.00%)
+Positive Passed: 86/86 (100.00%)

tasks/coverage/snapshots/formatter_misc.snap

@@ -1,3 +1,3 @@
 formatter_misc Summary:
-AST Parsed     : 85/85 (100.00%)
-Positive Passed: 85/85 (100.00%)
+AST Parsed     : 86/86 (100.00%)
+Positive Passed: 86/86 (100.00%)

tasks/coverage/snapshots/lexer_misc.snap

@@ -1,3 +1,3 @@
 lexer_misc Summary:
-AST Parsed     : 270/270 (100.00%)
-Positive Passed: 270/270 (100.00%)
+AST Parsed     : 273/273 (100.00%)
+Positive Passed: 273/273 (100.00%)

tasks/coverage/snapshots/parser_misc.snap

@@ -1,7 +1,7 @@
 parser_misc Summary:
-AST Parsed     : 85/85 (100.00%)
-Positive Passed: 85/85 (100.00%)
-Negative Passed: 185/185 (100.00%)
+AST Parsed     : 86/86 (100.00%)
+Positive Passed: 86/86 (100.00%)
+Negative Passed: 187/187 (100.00%)
 
   × Cannot assign to 'arguments' in strict mode
    ╭─[misc/fail/arguments-eval-ambient.ts:2:13]
@@ -136,6 +136,28 @@ Negative Passed: 185/185 (100.00%)
     ╰────
   note: Classes are always strict mode code
 
+  × Only a single declaration is allowed in a `for...in` statement
+   ╭─[misc/fail/arrow-block-body-in-context.js:1:6]
+ 1 │ for (var f = () => {}, value = a in b;;);
+   ·      ───────────────────────────
+   ╰────
+
+  × Expected `)` but found `;`
+   ╭─[misc/fail/arrow-block-body-in-context.js:1:38]
+ 1 │ for (var f = () => {}, value = a in b;;);
+   ·     ┬                                ┬
+   ·     │                                ╰── `)` expected
+   ·     ╰── Opened here
+   ╰────
+
+  × Expected `)` but found `;`
+   ╭─[misc/fail/arrow-expression-body-in.js:1:26]
+ 1 │ for (var f = () => a in b;;);
+   ·     ┬                    ┬
+   ·     │                    ╰── `)` expected
+   ·     ╰── Opened here
+   ╰────
+
   × Illegal break statement
    ╭─[misc/fail/arrow-function-jumps.js:1:25]
  1 │ while (true) { (() => { break; }); }

tasks/coverage/snapshots/semantic_misc.snap

@@ -1,6 +1,6 @@
 semantic_misc Summary:
-AST Parsed     : 85/85 (100.00%)
-Positive Passed: 81/85 (95.29%)
+AST Parsed     : 86/86 (100.00%)
+Positive Passed: 82/86 (95.35%)
 semantic Error: tasks/coverage/misc/pass/declare-let-private.ts
 Bindings mismatch:
 after transform: ScopeId(0): ["private"]

tasks/coverage/snapshots/transformer_misc.snap

@@ -1,3 +1,3 @@
 transformer_misc Summary:
-AST Parsed     : 85/85 (100.00%)
-Positive Passed: 85/85 (100.00%)
+AST Parsed     : 86/86 (100.00%)
+Positive Passed: 86/86 (100.00%)