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

Issue number: resolves #29321

---------

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## What is the current behavior?
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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?
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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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## 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 {