* Validate flow CEL expressions at definition load time
Promote CEL expression handling to a public Expression API and validate expressions when a FlowDefinition is built instead of when it executes.
Invalid CEL syntax or unknown roots now raise ValidationError from FlowDefinition.from_yaml() and FlowDefinition.from_dict(). Expressions may reference state and outputs, plus item inside each.do; bare identifiers are rejected as unknown roots.
For with values, the CEL contract is intentionally simple: after trimming whitespace, a string is evaluated as CEL only if it starts with ${ and ends with }. Anything else is treated as a literal value, so partial interpolation is not supported. If the content inside the wrapper is not valid CEL, validation fails.
Examples:
```text
"${state.topic}" -> evaluated, returns state.topic
"topic is ${state.topic}" -> literal string
"${state.topic} suffix" -> literal string
"${'a'}${'b'}" -> invalid CEL
```
* Honor explicit empty-context overrides in evaluate() / render_template()
b0816e00b6