fix(button): preserve value on form reset

[kaigritun]

Summary

Fixes #2524

Problem

When a form containing <sl-button> elements with name/value attributes is reset, the button values were being cleared instead of being preserved.

For example:

<form>
  <sl-button type='submit' name='rtype' value='month'>List by month</sl-button>
  <sl-button type='reset'>Reset</sl-button>
</form>

After clicking the reset button, the submit button's value would become empty.

Root Cause

The FormControlController.handleFormReset() method calls setValue(host, defaultValue(host)) to restore form controls to their default values. However, sl-button didn't have a defaultValue property, so defaultValue(host) returned undefined, which was then set as the value.

Fix

Added a defaultValue property to sl-button that is initialized to the button's value in firstUpdated(). This ensures that:

1. The initial value from the HTML attribute is captured as the default
2. Form reset restores the button to this initial value

This matches the native HTML behavior where button values are fixed in the markup and don't change on form reset.

Testing

Added a test case to verify the button value is preserved after form reset.

[vercel]

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[KonnorRogers]

Funnily enough, native buttons don't have a defaultValue and are not subject to button resets, so they should just "stay as they are" and not try to revert.

There's a whole heuristic around what the browser does, but the TLDR is that: "if its user-editable IE: textarea, input, etc it has a value / defaultValue, for buttons, checkboxes, etc there is no defaultValue (checkboxes have defaultChecked and options have defaultSelected, but no defaultValue, its a fun time...)

[kaigritun]

Thanks for the context! You're right - I checked the spec and native <button> elements don't participate in form reset for their value attribute (only inputs, textareas, and select do).

So the correct fix would be to not reset the value at all, which means removing the formResetCallback() that sets this.value = '' rather than adding defaultValue.

Want me to update the PR to just remove that line instead? Or would you prefer to close this and handle it differently?

[KonnorRogers]

i dont believe shoelace has a formResetCallback(), instead, what I would do is modify this check here:

https://github.com/kaigritun/shoelace/blob/5c531816841935ae4caee706ba1c125a0a2de3c3/src/internal/form.ts#L255

to be more like:

-    this.options.setValue(this.host, this.options.defaultValue(this.host));
+    if ("defaultValue" in this.host) {
+      this.options.setValue(this.host, this.options.defaultValue(this.host));
+    }

and then remove the defaultValue property from sl-button. value should probably also reflect, but that isn't necessarily pertinent to this PR.