Fix histogram escape key handling in VS Code extension (#2468)

When users clicked "Histogram" in VS Code and were prompted for the
number of shots, pressing Escape did not cancel the operation as
expected. Instead, the histogram was still displayed with a default
value of 100 shots.

## Root Cause

The issue was in the `handleShowHistogram` function in
`webviewPanel.ts`. The code used a fallback pattern that prevented
proper escape key detection:

```typescript
const numberOfShots =
  (await window.showInputBox({
    value: "100",
    prompt: "Number of shots",
    validateInput: validateShotsInput,
  })) || "100";

// This check never worked because of the fallback above
if (numberOfShots === undefined) {
  return;
}
```

When a user pressed Escape, `showInputBox()` returned `undefined`, but
the `|| "100"` fallback immediately assigned "100" to `numberOfShots`,
making the subsequent `undefined` check never true.

## Solution

Removed the problematic fallback and properly checked for escape key
before using the input value:

```typescript
const numberOfShotsInput = await window.showInputBox({
  value: "100",
  prompt: "Number of shots",
  validateInput: validateShotsInput,
});

// Now this properly detects when user presses Escape
if (numberOfShotsInput === undefined) {
  return;
}

const numberOfShots = numberOfShotsInput;
```

This pattern matches how escape key handling is implemented correctly in
other parts of the codebase, such as the Azure workspace actions and the
resource estimation commands.

## Testing

- ✅ VS Code extension builds successfully
- ✅ TypeScript compilation passes
- ✅ Linting and formatting checks pass
- ✅ No unrelated files modified

Fixes #1872.

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---

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: minestarks <[email protected]>

Author: Copilot

vscode/src/webviewPanel.ts

@@ -104,18 +104,19 @@ export function registerWebViewCommands(context: ExtensionContext) {
         }
       };
 
-      const numberOfShots =
-        (await window.showInputBox({
-          value: "100",
-          prompt: "Number of shots",
-          validateInput: validateShotsInput,
-        })) || "100";
+      const numberOfShotsInput = await window.showInputBox({
+        value: "100",
+        prompt: "Number of shots",
+        validateInput: validateShotsInput,
+      });
 
       // abort if the user hits <Esc> during shots entry
-      if (numberOfShots === undefined) {
+      if (numberOfShotsInput === undefined) {
         return;
       }
 
+      const numberOfShots = numberOfShotsInput;
+
       sendMessageToPanel(
         { panelType: "histogram", id: panelId },
         true,