feat: add _meta support to CreateElicitationResult

[mzeng-openai]

Summary

• add optional _meta metadata to CreateElicitationResult
• add with_meta() builder and preserve None defaults at existing call sites
• cover _meta serialization/deserialization and refresh JSON-RPC schema snapshots

Tests

• cargo +nightly fmt --all --check
• cargo check -p rmcp --all-targets --all-features
• cargo check -p mcp-conformance --all-targets
• cargo test -p rmcp --test test_elicitation test_elicitation_result_serialization --features "elicitation client server"
• cargo test -p rmcp --test test_message_schema --all-features
• git diff --check