spec(draft): add missing HostCapabilities fields (updateModelContext and message)

[hydrosquall]

Motivation

The draft spec is the source of truth for contributors and implementors, including those who may not know to look in src/spec.types.ts for the full picture.

Two host capabilities present in the implementation were missing from the spec, making it less likely that new hosts would report these fields to client apps.

Changes

Adds two missing fields to HostCapabilities:

• updateModelContext - signals the host accepts ui/update-model-context requests to inject context into future model turns.
• message - signals the host accepts ui/message content from the view.

Both fields use a new SupportedContentBlockModalities helper interface (mirroring McpUiSupportedContentBlockModalities in spec.types.ts) that describes which content block types the host supports for each capability.

Testing

N/A

[hydrosquall]

Matches the ordering in the types

ext-apps/src/spec.types.ts

Lines 520 to 523 in 0008d3b

	/** @description Host accepts context updates (ui/update-model-context) to be included in the model's context for future turns. */
	updateModelContext?: McpUiSupportedContentBlockModalities;
	/** @description Host supports receiving content messages (ui/message) from the view. */
	message?: McpUiSupportedContentBlockModalities;

[hydrosquall]

See

ext-apps/src/spec.types.ts

Lines 462 to 475 in 0008d3b

	export interface McpUiSupportedContentBlockModalities {
	/** @description Host supports text content blocks. */
	text?: {};
	/** @description Host supports image content blocks. */
	image?: {};
	/** @description Host supports audio content blocks. */
	audio?: {};
	/** @description Host supports resource content blocks. */
	resource?: {};
	/** @description Host supports resource link content blocks. */
	resourceLink?: {};
	/** @description Host supports structured content. */
	structuredContent?: {};
	}

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commit: 8f97cd3