test(e2e): add Playwright coverage for the Collection Docs top bar

[lokesh-bruno]

Ref BRU-4631

Description

Adds E2E coverage for the Collection Docs top bar so the header controls, Show vars, and environment switching are checked in Playwright. Edge cases use ?fixture=general and the general-* collections. An unknown fixture name shows an error instead of the sample collection.

The coverage includes:

• Search, Show vars, and the environment dropdown on the desktop bar.
• Header controls staying visible without overlap, including the theme toggle.
• The bar staying put while Overview, a request, and Environments scroll.
• The environment menu listing only the included environments, with the first one selected.
• Selecting an environment updating the dropdown label, and the label staying in sync across a couple of switches.
• Show vars and the selected environment surviving refresh and navigation.
• A single environment in the dropdown, and "No environments" when none are included.
• A very long collection name truncating at tablet width without covering the other controls.
• Hiding the version in the bar and on Overview when the collection has none.
• Show vars resolving {{host}} in the URL, headers, body, and snippet, and putting the placeholders back when turned off.
• Secret values staying masked on those same surfaces.
• Show vars staying consistent across toggles.
• Resolved values updating on the same request and the next request without a reload.
• Resolved values staying in sync across a couple of environment switches, and after refresh and navigation.
• A variable missing from the active environment staying unresolved.
• Special-character values rendering as text.
• A single environment still resolving its variables.
• An empty Environments page when none are included.

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