feat(ext-mcp): add MCP (Model Context Protocol) server extension

Streamable HTTP transport (JSON-RPC 2.0 over POST), one-class-per-tool/resource/prompt
API mirroring RequestHandler, boot-time-precompiled schema/list payloads for a zero-alloc
hot path, and optional OAuth2 protection built on flash-ext-oidc (lazy-loaded, RFC 8707
audience binding, RFC 9728 Protected Resource Metadata). Registers the module in the
root and flash-extensions POMs and adds the ext-mcp commit scope to AGENTS.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flash-extensions/pom.xml

@@ -23,6 +23,7 @@
         <module>flash-ext-view-thymeleaf</module>
         <module>flash-ext-limiter</module>
         <module>flash-ext-web-bundler</module>
+        <module>flash-ext-mcp</module>
         <module>flash-ext-data-core</module>
         <module>flash-ext-data-jdbc</module>
         <module>flash-ext-data-hibernate</module>