JebProvider throws when REA_JEB_MCP_URL fails endpoint validation, and the
composition built it eagerly for the CLI and every MCP connection. A
malformed value, or http://localhost:8425/mcp, therefore crashed every CLI
command with a stack trace and failed every MCP connection with an
internal server error. The composition now parses the endpoint first and,
when it is unusable, returns a JEB port that reports not_configured
availability and a ConfigurationError naming REA_JEB_MCP_URL.
Fixes#2143
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>