feat: save page source on failed assertions

[jeremy-langevin-twc]

This PR adds an opt-in capability to save the full HTML source of the page when a browser assertion fails, alongside the failure screenshot, console logs, and JavaScript errors that are already captured:

// tests/Pest.php
pest()->browser()->source();

When enabled and a browser assertion fails, the page source is written to tests/Browser/Source/ and the failure message gains a line pointing to it, mirroring the existing screenshot line:

A screenshot of the page has been saved to [Tests/Browser/Screenshots/...]. The source of the page has been saved to [Tests/Browser/Source/...].

Why: a screenshot cannot show data-test attributes, hidden or collapsed elements, hydration state, or the actual markup a failing selector was matched against. Laravel Dusk stores page source on failure for exactly this reason, so this also closes a Dusk-parity gap.

Why default-off: rendered pages can be large, and page HTML can expose PII more structurally than a screenshot does.

Naming: source() follows the existing terse config grammar (debug(), diff(), headed()) and reuses the term the API already uses for page HTML (assertSourceHas()); the directory matches Dusk's tests/Browser/source in this repo's capitalization.

Behavior details:

• The source dump is skipped in the same cases the failure screenshot is skipped — debug mode and screenshot-diff failures — so both artifacts follow one predictable condition.
• Saving the source can never mask the original failure: it is wrapped so that an error in fetching the page content or writing the file leaves the assertion message (including the screenshot and console-log lines) intact.
• Source::cleanup() runs at suite boot exactly like Screenshot::cleanup(), and tests/Browser/Source is gitignored like the screenshots directory.
• Zero behavior change for anyone who doesn't call source().

This is a backwards-compatible, opt-in addition (SemVer minor). Covered by unit tests for the support class and browser tests for both the enabled and disabled failure paths; composer test passes.

[jeremy-langevin-twc]

Closing this is not ready