fix(core): correct range intersection when ranges share start/end lines

[eeshsaxena]

Description

intersection(a, b) in core/util/ranges.ts computes the wrong start/end character in the multi-line branch when the two ranges begin (or end) on the same line.

It picked the character from whichever range's line matched startLine/endLine first:

const startCharacter = startLine === a.start.line ? a.start.character : b.start.character;

When both ranges start on that shared line, this returns a's character instead of the later of the two — and symmetrically for the end. Example (both share start line 0 and end line 3):

intersection(
  { start: { line: 0, character: 5 },  end: { line: 3, character: 4 } },
  { start: { line: 0, character: 10 }, end: { line: 3, character: 2 } },
)
// before: { start: {0, 5},  end: {3, 4} }   ❌
// after:  { start: {0, 10}, end: {3, 2} }   ✔  (later start, earlier end)

The single-line branch already does this correctly (Math.max / Math.min); this makes the multi-line branch consistent.

Fix

When the two ranges share the start line, take Math.max of the start characters; when they share the end line, take Math.min of the end characters. Otherwise the previous behavior is unchanged.

Testing

Added a ranges.test.ts case for two ranges sharing both start and end lines (fails before, passes after). All existing intersection tests still pass (they use ranges on different start/end lines, so they don't touch the changed branch). Verified the logic directly against the existing test inputs plus the new case.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}