Update the import path from github.com/google/cel-go to cel.dev/cel-go
and fix two breaking changes.

NativeToValue now converts json.Number to Int or Double rather than
passing it through. Fix by converting json.Number to its string
representation before the call in all UseNumber decode paths.

The internal evaluator now calls Exec() on interpretable nodes via the
new InterpretableV2 interface. Update the coverage decorator to use
CustomDecoratorV2 and add Exec() overrides to all wrapper types so
coverage is recorded from either evaluation path. Update the
unsupported-type guard to use InterpretableV2 as its method-count
baseline.

Also bump go version to go1.26.8 to address govulncheck issues.