[Client] Fill elicitation defaults when accepting (SEP-1034)

Context

SEP-1034 adds client-side default handling for elicitation/create. When a client accepts an elicitation request without explicit field values, the response content should be auto-populated from default fields declared in the request's requestedSchema. Schemas already expose default on StringSchemaDefinition, NumberSchemaDefinition, EnumSchemaDefinition, TitledEnumSchemaDefinition, and BooleanSchemaDefinition.

Scope

• Add helper ElicitationSchema::extractDefaults(): array<string, mixed> returning a field => default map for all properties that declare one.
• Document the helper as the canonical "accept-with-defaults" path; update the example in tests/Conformance/client.php to use it for elicitation-sep1034-client-defaults.
• Optional convenience: ElicitResult::accepted(ElicitationSchema $schema) factory that pre-fills defaults.

Conformance scenarios unblocked

elicitation-sep1034-client-defaults.

Dependencies

Independent.

Acceptance

• Unit test on ElicitationSchema::extractDefaults covering all 5 definition types.
• Conformance: scenario passes (after #328 wiring lands, see below).

cc @soyuka

Start with ElicitationSchema and the five schema definition types that expose default values, then inspect tests/Conformance/client.php and the elicitation-sep1034-client-defaults scenario. Add the extractDefaults unit coverage and update the example to use the helper; done means all five types are covered and the conformance scenario passes.