fix(code-editor): adaptLSDiagnosticRange fails to expand empty range at EOF

[xgopilot]

Problem

In spx-gui/src/components/editor/code-editor/xgo-code-editor/diagnostics.ts, the adaptLSDiagnosticRange function is supposed to expand a zero-length diagnostic range so that it can be displayed as an inline decoration. However, when the diagnostic position is at or near EOF, the expansion logic fails:

// If no non-newline character is found forward, fall back to backward scan:
for (let i = offsetStart; i >= 0; i--) {
  if (code[i] !== '\n') {
    offsetStart = i
    break
  }
}

When offsetStart === code.length (EOF), the backward scan starts at index code.length, but code[code.length] is undefined (not '\n'), so offsetStart = i is set to code.length — leaving start === end. The expanded range is still empty.

Expected Behavior

adaptLSDiagnosticRange should always return a non-empty range (i.e. start !== end) when given a zero-length input range, including at EOF.

Suggested Fix

• Start the backward scan from offsetStart - 1 (not offsetStart) to avoid the off-by-one issue.
• When no non-newline character exists anywhere (e.g. file is all newlines or empty), ensure offsetEnd is advanced to at least offsetStart + 1 as a last resort.

Location

spx-gui/src/components/editor/code-editor/xgo-code-editor/diagnostics.ts — adaptLSDiagnosticRange method

Notes

This bug is pre-existing (not introduced by PR #2858). Tracked here per review comment in that PR.