CmdPal: Remove article from "Select a file" button text (#49752)

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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 PR changes file picker parameter button text from "Select a file"
to "Select file".

## Pictures? Pictures!

<img width="1600" height="953" alt="image"
src="https://github.com/user-attachments/assets/221eeceb-ac49-4469-8989-b8d01cb4dd2c"
/>


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## PR Checklist

- [ ] Closes: #xxx
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- [ ] **Communication:** I've discussed this with core contributors
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- [ ] **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
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
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and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/Properties/Resources.Designer.cs

@@ -115,7 +115,7 @@ namespace Microsoft.CommandPalette.Extensions.Toolkit.Properties {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to Select a file.
+        ///   Looks up a localized string similar to Select file.
         /// </summary>
         internal static string FilePickerParameterRun_PlaceholderText {
             get {

src/modules/cmdpal/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/Properties/Resources.resx

@@ -158,6 +158,6 @@
     <comment>{0} is the error message</comment>
   </data>
   <data name="FilePickerParameterRun_PlaceholderText" xml:space="preserve">
-    <value>Select a file</value>
+    <value>Select file</value>
   </data>
 </root>