fix(web): close model selector before opening provider dialogs - #14173

Description

Fixes the model selector popover remaining visually open when another model-related dialog is opened.

The ModelSelector now supports controlled open state, allowing the Language Models page to close the Default Model selector before opening a provider configuration dialog.

Fixes #14172

How Has This Been Tested?

• Manually verified the Default Model selector can be opened and closed normally.
• Verified that opening a provider configuration dialog closes the Default Model selector.
• Verified the stale, non-interactive dropdown no longer remains visible over the dialog.

Additional Options

• [Optional] Please cherry-pick this PR to the latest release version.
• [Optional] Override Linear Check

Summary by cubic

Closes the Default Model selector before opening any provider modal, preventing a stale popover from overlaying dialogs. Previously the popover stayed visible over provider config and delete dialogs; now the page controls the popover and closes it first.

• Adds optional open and onOpenChange props to ModelSelector; falls back to internal state when not provided.
• LanguageModelsPage tracks the selector’s open state and closes it via onBeforeOpen before opening Existing/New/Custom provider configuration modals and the delete modal.
• No breaking changes; existing uncontrolled usages continue to work.

^{Written for commit 8f4523c. Summary will update on new commits.}