Comment creation fails on GitLab <= 17.7

Posting a review comment / draft note fails with a `400 Bad Request` from the GitLab API:
```json
{"message": {"position": ["must be a valid json schema"]}}
```

**Environment**
- `gitlab.nvim` commit: `80edb27` / `f05aee3` (latest)
- GitLab version: `17.7.1` (self-hosted)

**Root Cause**
In commit `80edb27` (`fix: get correct location data for comments efficiently`), `OldLine` and `NewLine` were added to `opt.LineRange.Start` and `opt.LineRange.End` in `cmd/app/comment_helpers.go`:

```go
opt.LineRange = &gitlab.LineRangeOptions{
Start: &gitlab.LinePositionOptions{
Type: &positionData.LineRange.Start.Type,
LineCode: &startFilenameSha,
OldLine: &startOldLine,
NewLine: &startNewLine,
},
...
}
```

This causes the Go backend to serialize:
```json
"line_range": {
"start": {
"line_code": "...",
"type": "new",
"old_line": 0,
"new_line": 36
}
}
```

GitLab's REST API validates `position` against `app/validators/json_schemas/position.json`. In GitLab $\le$ 17.7, `line_range.start` and `line_range.end` enforce `"additionalProperties": false` and **only** permit `"line_code"` and `"type"`. As a result, including `old_line` or `new_line` causes schema validation to fail.

**Fix**
In `cmd/app/comment_helpers.go`, keep `OldLine` and `NewLine` nil / omitted from `LinePositionOptions`:

```diff
--- a/cmd/app/comment_helpers.go
+++ b/cmd/app/comment_helpers.go
@@ -83,14 +83,10 @@ func buildCommentPosition(commentWithPositionData RequestWithPosition) *gitlab.P
opt.LineRange = &gitlab.LineRangeOptions{
Start: &gitlab.LinePositionOptions{
Type: &positionData.LineRange.Start.Type,
LineCode: &startFilenameSha,
- OldLine: &startOldLine,
- NewLine: &startNewLine,
},
End: &gitlab.LinePositionOptions{
Type: &positionData.LineRange.End.Type,
LineCode: &endFilenameSha,
- OldLine: &endOldLine,
- NewLine: &endNewLine,
},
}
```

Start in cmd/app/comment_helpers.go at buildCommentPosition and inspect how LineRangeOptions is assembled for review comments and draft notes. Compare the serialized position with GitLab 17.7's accepted schema, then verify that comment creation no longer returns the 400 validation error.

Written by the indexing model from the issue text.