fix: allow theme-check-disable-next-line to suppress checks inside doc tags

[SinhSinhAn]

What is this PR?

This fixes a bug where {% # theme-check-disable-next-line %} placed before a {% doc %} tag doesn't actually suppress any of the checks that run against the doc tag's contents.

What was the issue?

Issue #945 reported that you can't use theme-check-disable-next-line to silence checks inside {% doc %} tags. For example, if you have a snippet with duplicate param names that you want to temporarily ignore:

{% # theme-check-disable-next-line UniqueDocParamNames %}
{% doc %}
  @param prods - products
  @param prods - intentional duplicate for some reason
{% enddoc %}

The UniqueDocParamNames offense would still fire despite the disable comment. The workaround was wrapping the entire doc tag in a disable/enable pair, which is verbose.

Why it was happening

The disable-next-line logic in findNextLinePosition has a guard for block tags: when the next node has a blockStartPosition (like {% if %}, {% for %}, or {% doc %}), it intentionally returns only the blockStartPosition span (just the opening marker, e.g., the literal {% doc %} characters) rather than the full tag body. This prevents disable-next-line from accidentally suppressing checks on all children of control flow tags.

The problem is that {% doc %} tags are different from {% if %} tags. Doc tag checks (like UniqueDocParamNames, ValidDocParamTypes, ReservedDocParamNames) report their offenses at positions inside the doc body (where the @param declarations live). Since blockStartPosition only covers the opening {% doc %} marker, those offense positions fall outside the disabled range and are never suppressed.

How this PR fixes it

When disable-next-line resolves the next node and that node is a LiquidRawTag with name === 'doc', it now returns the full position span (covering the entire tag from {% doc %} to {% enddoc %}) instead of just blockStartPosition.

This is safe because doc tags don't contain nested Liquid control flow. Their body is parsed by a separate grammar (LiquidDocGrammar) that only produces doc-specific nodes (@param, @description, @example). There's no risk of accidentally silencing unrelated checks on deeply nested children.

The change is a 4-line addition to findNextLinePosition in disabled-checks/index.ts.

Tests

Added 3 test cases to disabled-checks/index.spec.ts:

• Disable-next-line with a specific check name suppresses that check inside doc tags
• Disable-next-line without a check name suppresses all checks inside doc tags
• Without the disable comment, offenses inside doc tags are still reported (regression guard)

All 18 tests pass (15 existing + 3 new).

Closes #945