Add page-aware API version switcher for API Explorer

[reakaleek]

Why

Multi-version API pages need a version switcher that links to the same operation in another major when it exists there. The interim native select only navigated to version landing pages and did not match the docs navigation dropdown pattern.

What

Replace the interim <select> with the same focus-within dropdown used in docs navigation. Build switcher targets from the version index and a cross-version page index so operation, tag, and schema pages deep-link across majors with landing-page fallback. Hide the product #nav-dropdown on API pages so only the version switcher appears in the left nav.

Closes elastic/docs-eng-team#723

Made with Cursor

[reakaleek]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in c9d0fa9. The two conflicting files were:

• IsolatedBuildNavigationHtmlWriter.cs — combined the suppressNavigationDropdown parameter from this PR with main's fix that sources top-level nav items from siteRoot (ensuring the dropdown lists all sections even when rendering a nested island).
• ApiPagesNavRenderingTests.cs — dropped two unused using statements (Elastic.Documentation.Site.Navigation and Nullean.ScopedFileSystem) that this PR had added but main had removed.

[reakaleek]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in 346250e. The conflict was in OpenApiGenerator.cs — origin/main had refactored the per-product generation loop into a private GenerateProduct method using the now-removed ApiVersionSwitcher.Build, while this PR had the ApiVersionSwitcherContext + ApiCrossVersionPageIndex logic inline. The resolution adopts origin/main's GenerateProduct private method structure and updates its body to use ApiVersionSwitcherContext/ApiCrossVersionPageIndex from this PR. All 134 ApiExplorer tests pass.