Add clearer validation errors for empty or malformed routing CEL expressions

[deepintai]

Context

Routing rules in the governance plugin are CEL expressions. When a rule's expression is empty or obviously malformed, the failure today is generic and surfaces late, which makes routing misconfigurations hard to debug. A small, focused validation helper with friendly messages improves the operator experience without changing routing behavior for valid rules.

What to do

1. Add a helper in deepintshield_server/plugins/governance/routing.go:

   func validateRoutingExpression(expr string) error

   covering:
   • Empty or whitespace-only expression -> "CEL expression cannot be empty or whitespace-only".
   • No condition operator present (none of ==, !=, in, contains, etc.) -> "CEL expression must contain at least one condition operator (==, !=, in, contains, etc.)".
2. Call it before expression compilation in the evaluation path (e.g. within EvaluateRoutingRules / before validateCELExpression), so invalid rules fail fast with a clear message.
3. Keep existing behavior for valid expressions unchanged.

Acceptance criteria

• validateRoutingExpression added with the two checks and exact messages above.
• Invoked in the routing evaluation path before CEL compilation.
• 3-4 unit tests in routing_test.go asserting the error messages and that valid expressions pass.
• go test ./plugins/governance/... passes.

File pointers

• Routing engine + evaluation: deepintshield_server/plugins/governance/routing.go (EvaluateRoutingRules:79, extractRoutingVariables:312, validateCELExpression:408)
• Tests: deepintshield_server/plugins/governance/routing_test.go