Fix UI tests: correct expandTopic parameter order and CI workflow (#936)

Fixes TypeScript compilation errors in UI tests and resolves CI workflow
configuration issue.

## Changes Made

### 1. Fixed expandTopic parameter order in ui-tests.spec.ts
- Corrected 5 function calls from `expandTopic(page, 'path')` to
`expandTopic('path', page)`
- Function signature: `expandTopic(path: string, browser: Page)`
- Aligns with existing usage in all scenario files (showNumericPlot.ts,
publishTopic.ts, etc.)

### 2. Fixed CI workflow configuration
- Updated `.github/workflows/tests.yml` to checkout PR code instead of
base branch
- Added `ref: ${{ github.event.pull_request.head.sha }}` to all 4
checkout actions
- The `pull_request_target` event defaults to checking out the base
branch; this fix ensures CI tests the PR's code

## Root Cause

The CI workflow was testing the base branch (master) which still had the
wrong parameter order, while the PR had the correct fix. This caused CI
to report TypeScript errors even though the PR code was correct.

## Testing

- ✅ TypeScript compilation passes locally (`tsc` and `yarn build`)
- ✅ Parameter order matches function signature and codebase conventions
- ✅ CI workflow now correctly tests PR code
- ✅ All 4 CI jobs (test, ui-tests, demo-video, test-browser) will use
corrected code

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> 
> *This section details on the original issue you should resolve*
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> <issue_title>Fix tests</issue_title>
> <issue_description>- fix backend tests
> - fix UI tests</issue_description>
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- Fixes thomasnordquist/MQTT-Explorer#935

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Co-authored-by: thomasnordquist <7721625+thomasnordquist@users.noreply.github.com>

src/spec/ui-tests.spec.ts

@@ -68,7 +68,7 @@ describe('MQTT Explorer UI Tests', function () {
       // When: Connect and expand topic
       await connectTo('127.0.0.1', page)
       await sleep(2000)
-      await expandTopic(page, 'livingroom/lamp')
+      await expandTopic('livingroom/lamp', page)
 
       // Then: Should see lamp state
       const stateTopic = await page.locator('span[data-test-topic="state"]')
@@ -97,7 +97,7 @@ describe('MQTT Explorer UI Tests', function () {
       // When: Connect and expand topic
       await connectTo('127.0.0.1', page)
       await sleep(2000)
-      await expandTopic(page, 'kitchen/coffee_maker')
+      await expandTopic('kitchen/coffee_maker', page)
 
       // Then: JSON content should be visible (check for heater key)
       const valueDisplay = await page.locator('text="heater"')
@@ -119,7 +119,7 @@ describe('MQTT Explorer UI Tests', function () {
       // When: Connect and expand to nested topic
       await connectTo('127.0.0.1', page)
       await sleep(2000)
-      await expandTopic(page, 'livingroom/lamp/brightness')
+      await expandTopic('livingroom/lamp/brightness', page)
 
       // Then: Brightness topic should be visible and selected
       const brightnessTopic = await page.locator('span[data-test-topic="brightness"]')
@@ -148,7 +148,7 @@ describe('MQTT Explorer UI Tests', function () {
       await sleep(1000)
       await clearSearch(page)
       await sleep(500)
-      await expandTopic(page, 'kitchen/temperature')
+      await expandTopic('kitchen/temperature', page)
 
       // Then: Temperature topic should be visible
       const tempTopic = await page.locator('span[data-test-topic="temperature"]')
@@ -175,7 +175,7 @@ describe('MQTT Explorer UI Tests', function () {
       await sleep(1000)
       await clearSearch(page)
       await sleep(500)
-      await expandTopic(page, 'kitchen/lamp')
+      await expandTopic('kitchen/lamp', page)
 
       // Then: Lamp topic should be visible
       const lampTopic = await page.locator('span[data-test-topic="lamp"]')