fix: propagate stdio process exit during initialization

[DragonFSKY]

Summary

• Detect unexpected stdio process exit after the child process has started.
• Propagate that stdio exit to an in-progress initialization instead of waiting for the request timeout.
• Preserve generic transport exception behavior and add regression coverage for a stdio server that exits before initialize.

Fixes #959

Notes

This keeps the scope limited to process exits that happen after ProcessBuilder.start() succeeds and before initialization completes. It does not change startup failures or normal stdio shutdown behavior.

A stdio-specific McpStdioServerProcessExitException is used so this initialization fast-fail path does not apply to generic transport exceptions from other transports.

Test Plan

• ./mvnw -pl mcp-core,mcp-test spring-javaformat:apply
• ./mvnw -pl mcp-core test
• ./mvnw -pl mcp-test -am test -Dtest=StdioMcpClientInitializationFailureTests -Dsurefire.failIfNoSpecifiedTests=false
• ./mvnw -pl mcp-test -am test -Dtest=HttpClientStreamableHttpAsyncClientResiliencyTests#testPing -Dsurefire.failIfNoSpecifiedTests=false
• ./mvnw clean test (local run reached mcp-test and failed in HTTP/Testcontainers resiliency/security integration tests; I did not count the full suite as passing)