fix: clamp completion range for unclosed string literals

[maruthang]

Summary

Fixes microsoft/vscode#273226

Bug: HTML attribute value completions with unclosed quotes would extend the replace range past < characters, causing subsequent HTML tags to be deleted when accepting a completion.

Root Cause: When a quoted attribute value is not closed, valueContentEnd was set to the end of the scanner token, which could span across < characters into subsequent HTML content. The completion replace range then covered too much text.

Fix: When the closing quote is missing (unclosed string literal), clamp valueContentEnd to the current cursor offset so the replace range does not extend into subsequent HTML content.

Changes

• src/services/htmlCompletion.ts: Added an else branch in the attribute value range calculation to set valueContentEnd = offset when the closing quote is absent, preventing the replace range from spanning past the cursor.
• src/test/completion.test.ts: Added two regression tests verifying that completions inside unclosed attribute values do not delete subsequent </th> or <td> tags.

Testing

• Added regression tests in src/test/completion.test.ts that verify completions for unclosed string literals preserve subsequent HTML tags
• All 147 existing tests pass

[aeschli]

@maruthang Thanks!
I think these's still a problem if there's a quote somewhere later in the code.