[Dev UI] Replace bespoke css vars with Material 3 surface tokens

[MichaelDoyle]

Description

Bespoke background CSS variables (--config-options-background and --resource-attachment-background-color) are currently used across model configuration panels and message resource chips:

• model-config-expansion-panel (used by object-input and safety-settings) uses --config-options-background (#f4f3f5 in light mode, rgb(34, 37, 42) in dark mode) for the expanded container background.
• message.component.scss uses --resource-attachment-background-color (#f4f3f5 in light mode, rgb(34, 37, 42) in dark mode) for media/attachment preview chips inside message cards.

Instead of maintaining bespoke background tokens across :root and body[data-theme='dark'], these should be refactored to standard Material 3 surface container tokens (var(--mat-sys-surface-container)).

Affected Components & Styles

• src/styles/_mixins.scss (model-config-expansion-panel mixin)
• src/app/components/message/message.component.scss (.resource-attachment-container)
• src/styles/global.scss (--config-options-background and --resource-attachment-background-color definitions)
• src/app/components/custom-config/components/object-input/
• src/app/components/model-config/safety-settings/

Expected Behavior / Proposed Solution

1. Replace var(--config-options-background) in model-config-expansion-panel with var(--mat-sys-surface-container).
2. Replace var(--resource-attachment-background-color) in message.component.scss with var(--mat-sys-surface-container).
3. Remove both --config-options-background and --resource-attachment-background-color from src/styles/global.scss.
4. Verify that nested input fields and resource attachment chips maintain appropriate contrast against their parent surfaces in both light and dark themes.