Fix AP system prompt textbox display issue (#43486)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
This pull request refactors how the system prompt is managed and
displayed in the Advanced Paste settings UI. The main improvements
center around normalizing the system prompt value, ensuring placeholder
prompts are handled consistently, and updating the logic for switching
between advanced and simple AI modes. These changes help prevent
placeholder text from being mistakenly saved as a custom system prompt
and improve maintainability.

**System prompt normalization and placeholder handling:**

* Added static normalization for both `AdvancedAISystemPrompt` and
`SimpleAISystemPrompt`, and introduced logic to clear the system prompt
if it matches a placeholder value. This prevents default placeholder
text from being persisted as a custom prompt.
[[1]](diffhunk://#diff-14126907329c7fcd49dd33bab32283296c7dd68ddc3902163a482a3b3ce58d36R39-R40)
[[2]](diffhunk://#diff-14126907329c7fcd49dd33bab32283296c7dd68ddc3902163a482a3b3ce58d36R942-R982)
* Refactored event handlers to use the new normalization logic and
updated the placeholder switching mechanism when toggling between
advanced and simple AI modes.
* Ensured system prompt normalization is applied when saving provider
configuration, further enforcing consistent prompt handling.

**UI improvements:**

* Removed hard-coded placeholder text from the XAML, ensuring that the
placeholder is now dynamically set based on the current mode and
normalized prompt value.
* Updated the logic for setting the system prompt textbox placeholder to
use the new normalization and mode detection methods, improving clarity
and reducing potential user confusion.
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## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
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signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
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installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

Signed-off-by: Shawn Yuan (from Dev Box) <shuaiyuan@microsoft.com>

src/settings-ui/Settings.UI/SettingsXAML/Views/AdvancedPastePage.xaml

@@ -533,7 +533,7 @@
                             MinHeight="76"
                             HorizontalAlignment="Stretch"
                             AcceptsReturn="True"
-                            PlaceholderText="You are tasked with reformatting user's clipboard data. Use the user's instructions, and the content of their clipboard below to edit their clipboard content as they have requested it. Do not output anything else besides the reformatted clipboard content."
+                            Header="System prompt"
                             Text="{x:Bind ViewModel.PasteAIProviderDraft.SystemPrompt, Mode=TwoWay}"
                             TextWrapping="Wrap" />
                         <Grid