Function ValidateFlowGraph

Validates a flow graph and returns all issues found.

The following checks are performed:

1. No event blocks — the graph has no entry points.
2. Unconnected required data inputs — a non-optional data input with no connection.
3. Unconnected signal inputs — an execution block whose in signal has no connection and that is not an event block (entry point).
4. Data type mismatches — a data connection whose source richType is incompatible with the target richType.
5. Unreachable blocks — blocks not reachable from any event block via signal/data traversal.
6. Data dependency cycles — circular data-only connections that would cause infinite recursion.