Hide model picker in the floating omni chat input window

[Copilot]

Removes the model picker as we don't need that here.

Description

• Gate the model picker menu item: Added ChatContextKeys.inChatInputWindow.negate() to the when clause of OpenModelPickerAction's MenuId.ChatInput contribution in chatExecuteActions.ts. The floating window's scoped context sets inChatInputWindow to true, so the picker is hidden there while remaining available in all other chat surfaces (main chat view, editor inline, notebook, terminal).

when:
    ContextKeyExpr.and(
        // Hide the model picker in the floating chat input window
        ChatContextKeys.inChatInputWindow.negate(),
        // Hide the model picker while a delegation (continue in) target is pending
        ChatContextKeys.hasPendingDelegationTarget.negate(),
        ...
    )

The change is a declarative context-key gate consistent with the existing conditions in the same when block, reusing the inChatInputWindow key already bound in the omni window's scoped context service.

[Copilot]

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