Make session target picker responsive for all session types

[Copilot]

Description

Session target picker only showed icon-only view for Local session type in narrow layouts. Other types (Background, Cloud, Claude, etc.) still displayed full labels, causing clutter.

Changed renderLabel to hide labels for all session types when input editor width < 650px.

Before:

if (currentType !== AgentSessionProviders.Local || !this.pickerOptions.onlyShowIconsForDefaultActions.get()) {
    labelElements.push(dom.$('span.chat-input-picker-label', undefined, label));
}

After:

if (!this.pickerOptions.onlyShowIconsForDefaultActions.get()) {
    labelElements.push(dom.$('span.chat-input-picker-label', undefined, label));
}

All AgentSessionProviders have icons defined, making this safe. The onlyShowIconsForDefaultActions observable already tracks editor width automatically.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make session target view responsive: show only icon when view is narrow</issue_title>
<issue_description>

Describe the feature you'd like.

When the session target view is resized to a narrower width, it should automatically become responsive and display only the icon, hiding any additional text or details. This would improve usability in compact layouts and reduce visual clutter. The design is inspired by feedback from UX discussions, where users requested a more minimal interface when screen space is limited.

This change would make the session target picker more intuitive and convenient, especially when working with multi-column layouts or picker components similar to those found in calendar date pickers.

Additional Context

• Current behavior shows all content, even in narrow views, which can become overwhelming.
• Desired behavior: Responsive design so that only the icon is visible when space is constrained.
• Related Slack conversation highlighted that users want an uncluttered, compact UI for quick interactions.

View original Slack conversation</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Make session target view responsive: show only icon when view is narrow #295033

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