fix(migrations): correctly detect then/else keywords in control flow migration

The control flow migration determines whether an `*ngIf` uses a `then`
and/or `else` clause by regex matching the raw microsyntax string for
the literal keywords `then`/`else`. The regexes only checked that the
keyword was preceded by a non-word character, but not that it was
followed by one.

As a result, a template reference name that merely starts with `then`
(e.g. `else thenBlock`) or `else` was misidentified as the `then`/`else`
keyword itself. This caused the migration to take the wrong code path
(e.g. then+else instead of else-only), which in turn made
`getTemplateName()` compute a `slice(start, end)` with `start > end`,
producing an empty template name. That empty placeholder was never
resolved and was silently emitted as an invalid
`<ng-template [ngTemplateOutlet]=""></ng-template>`, dropping the
original template content without any warning.

Add a negative lookahead `(?![\w\d])` to both regexes so `then`/`else`
are only matched as whole keywords, not as a prefix of a longer
template reference name.

Fixes #69914

(cherry picked from commit 5ad8231397)

packages/core/schematics/ng-generate/control-flow-migration/ifs.ts

@@ -75,8 +75,15 @@ export function migrateIf(template: string): {
 }
 
 function migrateNgIf(etm: ElementToMigrate, tmpl: string, offset: number): Result {
-  const matchThen = etm.attr.value.match(/[^\w\d];?\s*then/gm);
-  const matchElse = etm.attr.value.match(/[^\w\d];?\s*else/gm);
+  // The negative lookahead (?![\w$]) ensures `then`/`else` are matched only as
+  // whole keywords. Without it, an else/then template reference name that merely
+  // *starts* with `then`/`else` (e.g. `else thenBlock`) is misidentified as the
+  // `then` keyword, since `[^\w$];?\s*then` also matches the `then` prefix of
+  // `thenBlock`. `$` is included alongside `\w` (which already covers digits)
+  // since it is a valid identifier character in JS/template reference names
+  // (e.g. `#then$`), but is not part of `\w`.
+  const matchThen = etm.attr.value.match(/[^\w$];?\s*then(?![\w$])/gm);
+  const matchElse = etm.attr.value.match(/[^\w$];?\s*else(?![\w$])/gm);
 
   if (etm.thenAttr !== undefined || etm.elseAttr !== undefined) {
     // bound if then / if then else

packages/core/schematics/test/control_flow_migration_spec.ts

@@ -465,6 +465,125 @@ describe('control flow migration (ng update)', () => {
       );
     });
 
+    it('should migrate an if else case where the else template reference name starts with `then`', async () => {
+      writeFile(
+        '/comp.ts',
+        `
+        import {Component} from '@angular/core';
+        import {NgIf} from '@angular/common';
+
+        @Component({
+          templateUrl: './comp.html'
+        })
+        class Comp {
+          data: any;
+        }
+      `,
+      );
+
+      writeFile(
+        '/comp.html',
+        [
+          `<ng-container *ngIf="data.teamMember; else thenBlock">`,
+          `  <h2>Hello team member!</h2>`,
+          `</ng-container>`,
+          `<ng-template #thenBlock>`,
+          `  <h2>No team member</h2>`,
+          `</ng-template>`,
+        ].join('\n'),
+      );
+
+      await runMigration();
+      const content = tree.readContent('/comp.html');
+
+      expect(content).toBe(
+        [
+          `@if (data.teamMember) {`,
+          `  <h2>Hello team member!</h2>`,
+          `} @else {`,
+          `  <h2>No team member</h2>`,
+          `}\n`,
+        ].join('\n'),
+      );
+    });
+
+    it('should migrate an if then case where the then template reference name starts with `else`', async () => {
+      writeFile(
+        '/comp.ts',
+        `
+        import {Component} from '@angular/core';
+        import {NgIf} from '@angular/common';
+
+        @Component({
+          templateUrl: './comp.html'
+        })
+        class Comp {
+          data: any;
+        }
+      `,
+      );
+
+      writeFile(
+        '/comp.html',
+        [
+          `<ng-container *ngIf="data.teamMember; then elseyBlock"></ng-container>`,
+          `<ng-template #elseyBlock>`,
+          `  <h2>Then content</h2>`,
+          `</ng-template>`,
+        ].join('\n'),
+      );
+
+      await runMigration();
+      const content = tree.readContent('/comp.html');
+
+      expect(content).toBe(
+        [`@if (data.teamMember) {`, `  <h2>Then content</h2>`, `}\n`].join('\n'),
+      );
+    });
+
+    it('should migrate an if then else case where template reference names start with `then`/`else`', async () => {
+      writeFile(
+        '/comp.ts',
+        `
+        import {Component} from '@angular/core';
+        import {NgIf} from '@angular/common';
+
+        @Component({
+          templateUrl: './comp.html'
+        })
+        class Comp {
+          data: any;
+        }
+      `,
+      );
+
+      writeFile(
+        '/comp.html',
+        [
+          `<ng-container *ngIf="data.teamMember; then thenyBlock; else elseyBlock"></ng-container>`,
+          `<ng-template #thenyBlock>`,
+          `  <h2>Then content</h2>`,
+          `</ng-template>`,
+          `<ng-template #elseyBlock>`,
+          `  <h2>Else content</h2>`,
+          `</ng-template>`,
+        ].join('\n'),
+      );
+
+      await runMigration();
+      const content = tree.readContent('/comp.html');
+
+      expect(content).toBe(
+        [
+          `@if (data.teamMember) {`,
+          `  <h2>Then content</h2>`,
+          `} @else {`,
+          `  <h2>Else content</h2>`,
+          `}\n`,
+        ].join('\n'),
+      );
+    });
+
     it('should migrate an if case on a container', async () => {
       writeFile(
         '/comp.ts',