Fix parsing error propagation in Streamable HTTP JSON responses

[kele5555]

An HTTP 200 response containing invalid JSON currently completes sendMessage before parsing the body. The later parsing error cannot reach the pending request, so initialization eventually reports a timeout.

This moves the success signal after JSON-RPC parsing. Parsing failures now retain their cause and reach the existing McpClientSession failure callback, which removes the pending request. Notifications keep their existing behavior.

The existing empty-response test was named and documented as a notification test but sent an initialize request. It now sends notifications/initialized.

Validation on Java 17, with both Jackson 2 and Jackson 3:

• 82 related tests passed per profile, including 10 new regression cases.
• The initial regression run failed on the original code for transport completion, pending-request failure, and initialization failure.
• Tests cover malformed/empty/blank JSON, the parsing cause, pending cleanup before close, normal initialize/list/call, notifications, HTTP 503, connection refusal, and a real timeout.
• Docker-based tests were not run successfully in this environment; the full suite is not claimed as verified.

Related: #685 concerns SSE comment parsing. This change addresses the application/json response branch.

AI-assisted implementation, submitted for human review.

Fixes #1147