Add password input tests to TagHelperModelBindingTests

[Copilot]

Adds integration tests for password input tag helper model binding, mirroring the existing text input test coverage.

Changes

• Test methods: PasswordInput_RendersCorrectly and PasswordInputOverridden_RendersCorrectly validate model binding, metadata display (name, description), error messages, and property overrides
• Views: PasswordInput.cshtml and PasswordInputOverridden.cshtml demonstrating basic and overridden usage of <govuk-password-input>
• Model & controller: PasswordInputTestsModel record and corresponding action methods

[Fact]
public async Task PasswordInput_RendersCorrectly()
{
    var page = await fixture.Browser!.NewPageAsync();
    await page.GotoAsync($"{ServerFixture.BaseUrl}/ModelBindingTests/PasswordInput");

    var input = page.Locator("input[type='password']").First;
    Assert.Equal("Password", await input.GetAttributeAsync("name"));
    Assert.Equal("Model value", await input.GetAttributeAsync("value"));
    // ... validates label, hint, and error message from model metadata
}

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