Replace plugin Action callback with NativeInputHost

[malmstein]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1214157224317277/task/1214636594411870?focus=true

Description

Splits the interface part out of #8488 so it can land independently of the per-tab persistence work.

NativeInputPlugin.createView previously took an opaque (Action) -> Unit callback whose only payload was Action.StartChat. That shape didn't extend cleanly: every new plugin → host signal needed another sealed subclass, and plugins had no way to read the host's state at all.

This PR replaces the callback with a NativeInputHost interface exposing submit() and getInputState(). The host (NativeInputModeWidget) implements it and passes itself to plugins.

• StartChatNativeInputPlugin now calls host.submit() instead of onAction(Action.StartChat).
• ModelPickerNativeInputPlugin takes the new param without using host yet — the persistence PR will use host.getTabId() once that field is added.
• Action (sealed class) is removed; PromptContribution is unchanged.

The persistence/provider track (#8488) will rebase on this and add getTabId() to NativeInputHost, deprecate getPromptContribution(), and wire MutableNativeInputStateProvider.

Steps to test this PR

Start chat icon

• Tap the start-chat icon with no text — should open a new chat session
• Tap the start-chat icon with a query — should submit it as a chat message

Model picker

• Open the model picker, select a model, send a chat — selected modelId should still be applied (no behaviour change vs develop)

UI changes

Before	After
(No UI changes)	(No UI changes)

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Note

Medium Risk
Medium risk because it changes the plugin↔host communication contract and rewires submit/attachment signals, which could affect core input/attachment UX if any plugin or host path is missed.

Overview
Replaces the native input plugin callback-based Action API with a typed NativeInputHost interface, so plugins can invoke host behaviour (e.g. submit, attachment chooser/state updates) and query getInputState().

Updates NativeInputModeWidget to implement NativeInputHost and pass itself into NativeInputPlugin.createView, and migrates the start-chat and attachment flows (including AttachmentView notifications) to call host methods instead of emitting Action events. Tests and plugins are adjusted to the new signature; PromptContribution behaviour remains unchanged.

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[malmstein]

• Replace plugin Action callback with NativeInputHost #8498 👈 (View in Graphite)
• develop

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[joshliebe]

Looks good, let’s get this merged so me can migrate to using this pattern