fix(amazonq): edit diff UI doesnt render correctly when the diff contains only space char or empty line (#7922)

## Problem
The suggestion indicates the empty line (L4) is edited and replaced with
suggestion content however there is no diff highlight. Reason behind is
because the function `diffWordsWithSpace` will not say the line is
removed (since it's be edited).
https://github.com/aws/aws-toolkit-vscode/blob/master/packages/amazonq/src/app/inline/EditRendering/svgGenerator.ts#L437-L441

<img width="1299" height="1292" alt="image"
src="https://github.com/user-attachments/assets/a2a07ba0-28fe-4f31-9b8a-37d5ef2ff223"
/>






## Solution

<img width="1299" height="1292" alt="image"
src="https://github.com/user-attachments/assets/a8530917-ac06-4b9e-af39-ea037c38b340"
/>



---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Will-ShaoHua

packages/amazonq/src/app/inline/EditRendering/svgGenerator.ts

@@ -18,6 +18,8 @@ export const emptyDiffSvg = {
     originalCodeHighlightRange: [],
 }
 
+const defaultLineHighlightLength = 4
+
 export class SvgGenerationService {
     /**
      * Generates an SVG image representing a code diff
@@ -431,8 +433,12 @@ export class SvgGenerationService {
         for (let lineIndex = 0; lineIndex < originalCode.length; lineIndex++) {
             const line = originalCode[lineIndex]
 
+            /**
+             * If [line] is an empty line or only contains whitespace char, [diffWordsWithSpace] will say it's not an "remove", i.e. [part.removed] will be undefined,
+             * therefore the deletion will not be highlighted. Thus fallback this scenario to highlight the entire line
+             */
             // If line exists in modifiedLines as a key, process character diffs
-            if (Array.from(modifiedLines.keys()).includes(line)) {
+            if (Array.from(modifiedLines.keys()).includes(line) && line.trim().length > 0) {
                 const modifiedLine = modifiedLines.get(line)!
                 const changes = diffWordsWithSpace(line, modifiedLine)
 
@@ -455,7 +461,7 @@ export class SvgGenerationService {
                 originalRanges.push({
                     line: lineIndex,
                     start: 0,
-                    end: line.length,
+                    end: line.length ?? defaultLineHighlightLength,
                 })
             }
         }