fix(codegen): drop redundant space after export default when minifying (#23024)

## What

Found with a self-written whitespace analyzer (for each space in oxc's minified output, remove it and check the result still canonicalizes to the same AST — if so the space was non-optimal). The most common hit:

`ExportDefaultDeclaration` printed `"export default "` with an unconditional trailing space, kept before non-identifier values when minifying:

| Input | Before | After |
| --- | --- | --- |
| `export default {a:1}` | `export default {a:1}` | `export default{a:1}` |
| `export default [1]` | `export default [1]` | `export default[1]` |
| `export default (a,b)` | `export default (a,b)` | `export default(a,b)` |
| `export default "s"` | `export default "s"` | ``export default`s` `` |

## Fix

Use a soft space (kept when pretty-printing) and let the value insert a hard space only before an identifier-like token — matching how `return` already works, and esbuild's `SExportDefault` printer (`print("export default")` + `printSpace()`).

Spaces that are required are preserved: `export default function(){}`, `export default 5`, `export default foo`, `export default class C{}`.

### TypeScript

Exposed a latent bug: `TSInterfaceDeclaration` printed `interface` without a `print_space_before_identifier()` guard, so `export default interface I {}` would have minified to `export defaultinterface`. Added the guard (same fix as #23013 for enums). `export default interface`, `export default abstract class`, and `export default <const>x` all verified to round-trip.

Pretty-print output is unchanged. Codegen (115) and minifier (506) suites pass.

crates/oxc_codegen/src/gen.rs

@@ -1216,7 +1216,8 @@ impl Gen for ExportDefaultDeclaration<'_> {
         }
         p.print_indent();
         p.add_source_mapping(self.span);
-        p.print_str("export default ");
+        p.print_str("export default");
+        p.print_soft_space();
         self.declaration.print(p, ctx);
     }
 }
@@ -3930,6 +3931,7 @@ impl Gen for TSTypeAliasDeclaration<'_> {
 
 impl Gen for TSInterfaceDeclaration<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
+        p.print_space_before_identifier();
         if self.declare {
             p.print_str("declare ");
         }

crates/oxc_codegen/tests/integration/js.rs

@@ -45,6 +45,13 @@ fn module_decl() {
         "export { default } from './foo.js' with { type: 'json' }",
         "export{default}from\"./foo.js\"with{type:\"json\"};",
     );
+
+    test_minify("export default {a:1}", "export default{a:1};");
+    test_minify("export default [1]", "export default[1];");
+    test_minify("export default (a,b)", "export default(a,b);");
+    test_minify("export default 5", "export default 5;");
+    test_minify("export default foo", "export default foo;");
+    test_minify("export default function(){}", "export default function(){}");
 }
 
 #[test]

crates/oxc_codegen/tests/integration/ts.rs

@@ -181,6 +181,17 @@ export import b = require("b");
     snapshot_options("minify", &cases, &CodegenOptions::minify());
 }
 
+#[test]
+fn minify_export_default() {
+    let min = |src: &str, expected: &str| {
+        test_options_with_source_type(src, expected, SourceType::ts(), CodegenOptions::minify());
+    };
+    // A leading `interface`/`abstract`/keyword needs the space; `{`/`<` does not.
+    min("export default interface I { x: number }", "export default interface I{x:number;}");
+    min("export default abstract class {}", "export default abstract class{}");
+    min("export default <const>x;", "export default<const>x;");
+}
+
 #[test]
 fn ts_as_expression_in_binary_expr() {
     test_idempotency("key in (that as object)");