[Server][Capability] SEP-1865 MCP Apps: ext-apps registration, builder API, #[McpUiResource] attribute #351

Implements the server-side capability/builder portion of SEP-1865 (MCP Apps) for the MCP Spec 2026-07-28 release.

Tracked by umbrella #335. Depends on SEP-2133 + SEP-1865 schema sub-issue.

PHP SDK changes

• Register the ext-apps extension via SEP-2133 plumbing; declare server support during initialize and gate behavior on negotiation.
• Add Server::builder()->addUiResource(string $uri, string $html, array $metadata) to the public builder API.
• Add a discovery attribute #[McpUiResource(uri: 'ui://widget/foo')] parallel to #[McpResource], under src/Capability/Attribute/ and src/Capability/Registry/.
• New UiResourceReference and corresponding handler for resources/read on ui:// resources, ensuring the right MIME profile and _meta (sandbox hints, CSP, etc.) are returned.

Related

• Umbrella [2026-07-28] Extensions framework + Tasks + MCP Apps #335
• Tool↔UI linkage in separate issue
• Optional follow-up: examples/server/apps/ walkthrough + inspector tests