TUnit.Playwright: support a run-level headed switch (HEADED / BROWSER env or CLI option) #7285

Problem

TUnit.Playwright has no run-level switch for headed browsers. BrowserFixture.GetLaunchOptions() and BrowserTest use TUnitPlaywrightSettings.Default.DefaultBrowserTypeLaunchOptions ?? new BrowserTypeLaunchOptions() (BrowserFixture.cs:21-22, BrowserTest.cs:8), and nothing in the package reads an environment variable or command-line option for headed mode or browser choice.

So switching between headless CI and a visible local debug session means a code change, or a hand-written policy in every suite. Teams usually end up with something like Headless = !Debugger.IsAttached in a global settings hook. That silently ignores any documented environment switch and launches headless when a developer runs from a terminal.

The Microsoft Playwright .NET test adapters (NUnit and MSTest) honor HEADED=1 and BROWSER=<name> for this reason, so the same dotnet test invocation can run headed locally and headless in CI. Playwright itself also honors PWDEBUG=1, but only when the launch options leave Headless unset.

Proposed change

• Read HEADED (and optionally BROWSER) as the default when the user has not set Headless or overridden BrowserName, matching the Microsoft adapters.
• Alternatively, or in addition, add a TUnit command-line option such as --playwright-headed.
• Document the precedence: explicit BrowserTypeLaunchOptions.Headless wins, then the switch, then Playwright's default.
• Document that setting Headless explicitly overrides PWDEBUG=1.

Expected behaviour

HEADED=1 dotnet run --project Tests opens visible browsers for every BrowserTest, PageTest, BrowserFixture and PageFixture consumer with no code change. CI without the variable stays headless.