Message-level CEL can't compare two google.protobuf.Timestamp fields

[LuKev]

Ordering two Timestamp fields in a (buf.validate.message).cel rule doesn't transpile. The plugin emits an unconditional __cel_runtime_error__, so validate() returns Err for every input.

option (buf.validate.message).cel = {
  id: "time_range_query.ordered"
  expression: "!has(this.start_timestamp) || !has(this.end_timestamp) || this.start_timestamp < this.end_timestamp"
};

Cause: emit/cel.rs::field_to_schema_entry maps every FieldKind::Message to CelType::Dyn, so this.start_timestamp types as MessageRef("google.protobuf.Timestamp") and never reaches the (CelType::Timestamp, CelType::Timestamp) comparison arm in emit/cel_compile.rs. has() compiles fine; only the comparison falls back. scalar_this_for_with, in the same file, already does the WKT → CelType::Timestamp mapping for the field-level this binding.

Ask: map Timestamp and Duration message fields to CelType::Timestamp / CelType::Duration in the message-level schema path, leaving has() on presence semantics.

Reproduced on v0.7.0.