feat(forms): support binding number|null to <input type="text">

`<input type="number">` often does not provide the desired user experience when editing numbers in
a form. MDN even [describes](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input/number#using_number_inputs)
how text inputs should be used in many cases instead, via `<input type="text" inputmode="numeric">`
or similar configurations. Previously, this did not work with Signal Forms without a custom input
component/directive.

This PR builds support for binding `number|null` models directly to `<input type="text">` native
controls via `[formField]`. When a model has a number or `null` value, signal forms will preserve
that status when the user makes edits/changes. Empty string values are converted to `null`, other
values are parsed as numbers, and a parse error is raised when a non-numeric value is entered.

Note that it's up to the UI developer to configure additional UI affordances such as setting an
appropriate `inputmode`, rejecting non-numeric keypresses, etc.

Fixes #66903
Fixes #66157

packages/compiler-cli/test/ngtsc/signal_forms_spec.ts

@@ -106,7 +106,7 @@ runInEachFileSystem(() => {
       expect(extractMessage(diags[0])).toBe(`Type 'null' is not assignable to type 'string'.`);
     });
 
-    it('should infer an input without a `type` as a string field', () => {
+    it('should reject a numeric field without null on a bare input', () => {
       env.write(
         'test.ts',
         `
@@ -123,6 +123,120 @@ runInEachFileSystem(() => {
         `,
       );
 
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(extractMessage(diags[0])).toContain(
+        `is not assignable to type '{ (): string; set: (v: string) => void; } | { (): number | null; set: (v: number | null) => void; }'`,
+      );
+    });
+
+    it('should allow a string field on a bare input', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<input [formField]="f"/>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal(''));
+          }
+        `,
+      );
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(0);
+    });
+
+    it('should allow a number|null field on a text input', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<input type="text" [formField]="f"/>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal<number | null>(42));
+          }
+        `,
+      );
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(0);
+    });
+
+    it('should reject a string|null field on a text input', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<input type="text" [formField]="f"/>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal<string | null>(null));
+          }
+        `,
+      );
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(extractMessage(diags[0])).toContain(
+        `is not assignable to type '{ (): string; set: (v: string) => void; } | { (): number | null; set: (v: number | null) => void; }'`,
+      );
+    });
+
+    it('should reject a boolean field on a text input', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<input type="text" [formField]="f"/>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal(true));
+          }
+        `,
+      );
+
+      const diags = env.driveDiagnostics();
+      expect(diags.length).toBe(1);
+      expect(extractMessage(diags[0])).toContain(
+        `is not assignable to type '{ (): string; set: (v: string) => void; } | { (): number | null; set: (v: number | null) => void; }'`,
+      );
+    });
+
+    it('should reject a number field on a textarea', () => {
+      env.write(
+        'test.ts',
+        `
+          import {Component, signal} from '@angular/core';
+          import {FormField, form} from '@angular/forms/signals';
+
+          @Component({
+            template: '<textarea [formField]="f"></textarea>',
+            imports: [FormField]
+          })
+          export class Comp {
+            f = form(signal(0));
+          }
+        `,
+      );
+
       const diags = env.driveDiagnostics();
       expect(diags.length).toBe(1);
       expect(extractMessage(diags[0])).toBe(`Type 'number' is not assignable to type 'string'.`);