Add data-testid attribute to Button components (#1118)

* feat: add data-testid attributes for E2E testing

To improve E2E test stability and enable better component selection, this change adds `data-testid` attributes to several key button elements.

Since a generic `Button.tsx` component does not exist, the attributes were added directly to Material-UI `Button` components in the following locations:

- HR Zone buttons in `app/client/mock/page.tsx`
- Streaming `START`/`STOP` buttons in `app/client/mock/page.tsx`

This provides stable selectors for automated testing tools to target these critical interactive elements.

* feat: add data-testid attributes for E2E testing

To improve E2E test stability and enable better component selection, this change adds `data-testid` attributes to several key button elements.

Since a generic `Button.tsx` component does not exist, the attributes were added directly to Material-UI `Button` components in the following locations:

- HR Zone buttons in `app/client/mock/page.tsx`
- Streaming `START`/`STOP` buttons in `app/client/mock/page.tsx`

This provides stable selectors for automated testing tools to target these critical interactive elements.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

app/client/mock/page.tsx

@@ -163,6 +163,7 @@ export default function MockPage() {
                 variant="contained"
                 sx={{ backgroundColor: '#9E9E9E' }}
                 onClick={() => setHrByZone('grey')}
+                data-testid="zone-1-button"
               >
                 Zone 1
               </Button>
@@ -173,6 +174,7 @@ export default function MockPage() {
                 variant="contained"
                 sx={{ backgroundColor: '#2196F3' }}
                 onClick={() => setHrByZone('blue')}
+                data-testid="zone-2-button"
               >
                 Zone 2
               </Button>
@@ -183,6 +185,7 @@ export default function MockPage() {
                 variant="contained"
                 sx={{ backgroundColor: '#4CAF50' }}
                 onClick={() => setHrByZone('green')}
+                data-testid="zone-3-button"
               >
                 Zone 3
               </Button>
@@ -193,6 +196,7 @@ export default function MockPage() {
                 variant="contained"
                 sx={{ backgroundColor: '#FFEB3B', color: 'black' }}
                 onClick={() => setHrByZone('yellow')}
+                data-testid="zone-4-button"
               >
                 Zone 4
               </Button>
@@ -203,6 +207,7 @@ export default function MockPage() {
                 variant="contained"
                 sx={{ backgroundColor: '#F44336' }}
                 onClick={() => setHrByZone('red')}
+                data-testid="zone-5-button"
               >
                 Zone 5
               </Button>
@@ -218,6 +223,9 @@ export default function MockPage() {
             startIcon={<HeartBroken />}
             fullWidth
             sx={{ mb: 3 }}
+            data-testid={
+              isStreaming ? 'streaming-stop-button' : 'streaming-start-button'
+            }
           >
             {isStreaming
               ? `STOP Streaming HR: ${hrValue} BPM`