feat: add duplicateStructuredContent option to server builders

[neerajbhatt]

Summary

• Add duplicateStructuredContent(boolean) builder method to all server specifications (async, sync, stateless async, stateless sync)
• When true (default), structured content is auto-duplicated into a TextContent block in the content field for backwards compatibility — preserving existing behavior
• When false, the duplication is skipped, reducing response payload size for clients that fully support structuredContent
• Schema validation of structured output is always performed regardless of this setting

Usage

McpServer.async(transportProvider)
    .duplicateStructuredContent(false) // skip backwards-compat duplication
    .tool(myTool, myHandler)
    .build();

Changes

• McpServer.java — Added duplicateStructuredContent field and builder method to AsyncSpecification, SyncSpecification, StatelessAsyncSpecification, and StatelessSyncSpecification; passed through to server constructors
• McpAsyncServer.java — Added field, updated constructors and StructuredOutputCallToolHandler to conditionally duplicate
• McpStatelessAsyncServer.java — Same changes as McpAsyncServer

Test plan

• All 315 mcp-core unit tests pass
• HttpServletStatelessIntegrationTests structured output tests pass (1 pre-existing flaky testToolCallSuccess timeout)
• CI pipeline passes

Closes #688