fix: applies-switch and tab-set first tab hidden when block contains a comment

[Mpdreamz]

Why

Any non-item block inside {applies-switch} or {tab-set} — such as a MyST % comment — occupies index 0 in Markdig's child list, pushing all {applies-item} / {tab-item} blocks to indices 1, 2, 3 …

The Razor templates use Model.Index == 0 to emit checked="checked" on the first radio input, which is what the pure-CSS tab mechanism relies on to show the first panel. When Index is never 0, no radio is checked, and all tab content stays hidden until the user clicks a tab.

The real-world trigger was a % TODO comment in the query-logs source:

::::::{applies-switch}

% TODO: Update ECH and ECE tabs when Cloud UI supports configuring query logging settings directly.

:::::{applies-item} ess:
…

This caused #3254.

What

Changed AppliesItemBlock and TabItemBlock to compute their index relative to sibling items only, rather than among all children of the parent block:

// Before — breaks when any non-item block precedes the first item
Index = Parent!.IndexOf(this);

// After — always yields 0, 1, 2 … regardless of other block types
Index = Parent!.OfType<AppliesItemBlock>().ToList().IndexOf(this);

Added a regression test (ApplicabilitySwitchWithCommentTests) that places a % comment before the first {applies-item} and asserts both that indices are 0-based and that the first radio renders with checked="checked".

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 6068b4a6-8606-4e65-87b5-e4a597f82441

📥 Commits

Reviewing files that changed from the base of the PR and between b31505c and 555a521.

📒 Files selected for processing (3)

• src/Elastic.Markdown/Myst/Directives/AppliesSwitch/AppliesSwitchBlock.cs
• src/Elastic.Markdown/Myst/Directives/Tabs/TabSetBlock.cs
• tests/Elastic.Markdown.Tests/Directives/ApplicabilitySwitchTests.cs

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch fix/tab-rendering

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