fix: clamp completion range when scanner crosses tag boundary

[maruthang]

Summary

Follow-up to #239 for the same issue microsoft/vscode#273226. That PR handled the case where the attribute value has no closing quote anywhere; @federicobrancasi reported that the bug still reproduces in 1.117.0 Insiders when the document contains an unrelated later quote.

Root cause

In htmlCompletion.ts the replace range uses valueEnd, which comes from the scanner. When the opening " of an attribute value has no matching close on the same logical context, the scanner continues across </tag> boundaries and latches onto the next " it finds — e.g. the opening " of a different title="..." attribute elsewhere in the document. At that point text[valueEnd - 1] === text[valueStart] is true, so my previous else clamp didn't trigger and the replace range spanned the entire intervening markup.

Accepting checkbox from a completion on

<th><input type="che|</th><p title="later"></p>

would produce

<th><input type="checkbox title="later"></p>

wiping out </th><p . The regression test <th><input type="che|</th><p title="later"></p> (added in this PR) fails on main with exactly that output; see the test output in the repro notes below.

Fix

Even when a matching closing quote is found, scan the stretch from the cursor to that quote. If any < appears in that region, treat the value as unclosed and clamp valueContentEnd to offset. Any legitimate attribute-value content containing < would already be HTML-escaped to <, so an unescaped < in that region is a reliable signal that the scanner crossed a tag boundary.

Changes

• src/services/htmlCompletion.ts — detect tag-boundary-crossing between cursor and alleged closing quote; clamp to offset in that case.
• src/test/completion.test.ts — three regression tests:
  • unclosed quote + stray " on the same line (exact federicobrancasi scenario)
  • unclosed quote + stray " on a later line
  • unclosed quote crossing a \n before the closing tag

Testing

• Verified the new regression test fails on main with the expected buggy output before the fix is applied.
• npm test — 155/155 pass, npm run lint clean.
• All existing completion tests (including the two from fix: clamp completion range for unclosed string literals #239) continue to pass.

Fixes microsoft/vscode#273226 (round two).

[aeschli]

use the constant in the htmlScanner

[maruthang]

Done in 0365fb7 — exported _LAN from htmlScanner.ts and use it in htmlCompletion.ts. All 155 tests still pass. PTAL.

[maruthang]

Friendly ping @aeschli — addressed the inline comment in 0365fb7 by exporting _LAN from htmlScanner.ts and importing it in htmlCompletion.ts (no more duplicated literal). All 155 tests still pass. Ready for another look whenever convenient. Thanks!