Summary

• Detect unexpected stdio child-process exit and report a typed exception with the command and exit code.
• Propagate that failure to an in-progress client initialization instead of waiting for the request timeout.
• Add one subprocess regression test for the reported behavior.

Fixes #959

Tests

• ./mvnw -pl mcp-core test — 385 tests passed
• ./mvnw -pl mcp-test -am -Dtest=StdioMcpClientInitializationFailureTests -Dsurefire.failIfNoSpecifiedTests=false test — 1 test passed
• ./mvnw -DskipTests package — all 11 modules built successfully
• git diff --check