Add elicitation support for server-to-client user input requests

[wachterjohannes]

Motivation and Context

This PR implements the elicitation feature from the MCP specification (2025-06-18), which allows servers to request additional information from users during tool execution via the elicitation/create method.

Elicitation enables interactive workflows where the server can:

• Ask for user preferences or choices
• Collect form data with validated fields
• Request confirmation before performing actions

This is particularly useful for tools that need dynamic user input that can't be determined upfront.

How Has This Been Tested?

• Unit tests: 87 new tests covering all elicitation classes
• PHPStan: No errors at configured level
• Integration testing: Tested with OpenCode PR #8243 which adds elicitation client support
  • Verified book_restaurant tool with multi-field form (number, date, enum)
  • Verified confirm_action tool with boolean confirmation
  • Verified collect_feedback tool with rating enum and optional comments
  • Tested accept, decline, and cancel user responses

Breaking Changes

None. This is a purely additive feature.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Files added

Schema definitions (src/Schema/Elicitation/):

• StringSchemaDefinition - String fields with optional format (date, email, uri), minLength, maxLength
• NumberSchemaDefinition - Integer/number fields with min/max validation
• BooleanSchemaDefinition - Boolean checkbox fields
• EnumSchemaDefinition - String enums with optional human-readable labels (enumNames)
• PrimitiveSchemaDefinition - Factory for deserializing schema definitions
• ElicitationSchema - Wrapper for the requestedSchema object

Request/Result (src/Schema/):

• Enum/ElicitAction - User response actions (accept, decline, cancel)
• Request/ElicitRequest - The elicitation/create request
• Result/ElicitResult - The client's response with convenience methods (isAccepted(), isDeclined(), isCancelled())

Server changes:

• ClientGateway::elicit() - Convenience method for sending elicitation requests
• InitializeHandler - Now stores client capabilities in session for capability checking

Example (examples/server/elicitation/):

• Complete working example with three tools demonstrating different elicitation patterns
• Includes capability checking for graceful fallback when client doesn't support elicitation

Protocol details

Request format:

{
  "method": "elicitation/create",
  "params": {
    "message": "Please provide your reservation details:",
    "requestedSchema": {
      "type": "object",
      "properties": {
        "party_size": { "type": "integer", "minimum": 1, "maximum": 20 },
        "date": { "type": "string", "format": "date" }
      },
      "required": ["party_size", "date"]
    }
  }
}

Response format:
{
  "action": "accept",
  "content": { "party_size": 4, "date": "2025-02-14" }
}

[wachterjohannes]

@chr-hertel @Nyholm i am not totally happy with the code - have to rework a bit in the next days - but maybe there is already some feedback about it :)