fix: Support form and url fields in Elicitation capability per 2025-11-25 spec

[rhtnr]

Summary

Fixes #724

Updates the Elicitation record in McpSchema.java to support form and url fields as specified in the 2025-11-25 MCP specification.

Changes

• Added nested Form and Url records to Elicitation with proper Jackson annotations
• Added no-arg constructor for backward compatibility with existing code
• Added elicitation(boolean form, boolean url) builder method for explicit control
• Added 7 comprehensive tests covering deserialization, serialization, and backward compatibility

Backward Compatibility

This change is fully backward compatible:

Scenario	Before	After
Deserialize {"elicitation":{}}	✅ Works	✅ Works
Deserialize {"elicitation":{"form":{}}}	❌ Fails	✅ Works
Deserialize {"elicitation":{"form":{},"url":{}}}	❌ Fails	✅ Works
builder().elicitation().build()	✅ Works	✅ Works
Serialize empty elicitation	{}	{}

Test plan

• All 7 new tests pass
• All 82 existing tests pass
• Verified deserialization of spec-compliant JSON with form/url fields
• Verified backward compatibility with empty elicitation object

[tzolov]

Thanks @rhtnr,
This seems a decent workaround until the 2025-11-25 is implemented

[overheadhunter]

@tzolov did you read the discussion in #724? We have a more general fix in #725 that goes beyond just adding the "known" and also ignores the "unknown" to avoid similar problems in the future.