fix(select): options are visible with fit-content width and fill outline (#29408)

Issue number: resolves #29321

---------

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issue. -->

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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

When using a select with `fill="outline"`, `interface="popover"` and a
width that fits the content of the options, the select options are not
visible. The hidden radio is covering the text of the radio text
options.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Select options are visible and no longer covered by the hidden radio
container

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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1. Describe the impact and migration path for existing applications
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  2. Update the BREAKING.md file with the breaking change.
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## Other information

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screenshots of how the component looks before and after the change. -->

Dev-build: `8.0.2-dev.11714165638.13e7dd5b`

Reproduction (issue): https://stackblitz.com/edit/angular-mndtkr
Reproduction (with dev-build):
https://stackblitz.com/edit/angular-mndtkr-d7wsnp

Reproduction steps:
1. Open the select
2. Observe: The popover displays the options, but the options are not
visible/readable to the user
3. Use the dev-build
4. Open the select
5. Observe: The popover displays the options and they are
visible/readable to the user (5, 10, 15).

core/src/components/select-popover/select-popover.md.scss

@@ -2,7 +2,11 @@
 @import "./select-popover.md.vars";
 
 ion-list ion-radio::part(container) {
-  opacity: 0;
+  display: none;
+}
+
+ion-list ion-radio::part(label) {
+  @include margin(0);
 }
 
 ion-item {

core/src/components/select/test/fill/select.e2e.ts

@@ -237,4 +237,37 @@ configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, screenshot, co
       await expect(notchCutout).toBeHidden();
     });
   });
+
+  test(title('select: fill outline: fit-content should display select options'), async ({ page }, testInfo) => {
+    testInfo.annotations.push({
+      type: 'issue',
+      description: 'https://github.com/ionic-team/ionic-framework/issues/29321',
+    });
+    await page.setContent(
+      `
+        <ion-select
+          fill="outline"
+          aria-label="Fruit"
+          value="apple"
+          interface="popover"
+          style="width: fit-content"
+        >
+          <ion-select-option value="5">5</ion-select-option>
+          <ion-select-option value="10">10</ion-select-option>
+          <ion-select-option value="15">15</ion-select-option>
+        </ion-select>
+      `,
+      config
+    );
+
+    const select = page.locator('ion-select');
+    const ionPopoverDidPresent = await page.spyOnEvent('ionPopoverDidPresent');
+
+    await select.click();
+    await ionPopoverDidPresent.next();
+
+    const selectPopover = page.locator('ion-select-popover');
+
+    await expect(selectPopover).toHaveScreenshot(screenshot(`select-fill-outline-fit-content`));
+  });
 });