Add client-side default application for elicitation schemas, matching the
behavior specified in SEP-1034 and implemented in the TypeScript SDK.

- Add applyDefaults field to Elicitation.Form capability record
- Add applyElicitationDefaults() in McpAsyncClient to fill missing fields
  from schema defaults when the client returns accept with incomplete content
- Add builder method elicitation(form, url, applyDefaults)
- Add unit tests for applyElicitationDefaults covering all primitive types
- Add schema serialization tests for applyDefaults capability
- Add integration test verifying end-to-end default application flow

Closes modelcontextprotocol#699

- Build new HashMap from content instead of mutating in-place, preventing
  UnsupportedOperationException with unmodifiable maps (Map.of())
- Add IllegalArgumentException guard when applyDefaults=true but form=false
- Add Javadoc documenting applyDefaults as SDK-level behavior flag
  consistent with TypeScript SDK design
- Add integration test for unmodifiable map content
- Fix assertion to use assertThatJson node check instead of broad contains
- Fix copyright year to 2024-2025