fix: impl AsCelValue for FieldMask, Timestamp, Duration WKTs

[jrandolf]

Summary

Field-level predefined CEL rules on google.protobuf.FieldMask, Timestamp, and Duration failed to compile because the codegen plugin emits ::protovalidate_buffa::cel::AsCelValue::as_cel_value(inner) for the field value, but no AsCelValue impl existed for those WKT structs.

For example a downstream proto like

extend buf.validate.FieldMaskRules {
  repeated string mutable_paths = 80000001 [
    (buf.validate.predefined).cel = {
      id: "field_mask.mutable_paths"
      message: "update_mask paths must be one of the allowed values"
      expression: "this.paths.all(p, rule.exists(a, p == a || p.startsWith(a + '.')))"
    }
  ];
}

// ...

google.protobuf.FieldMask update_mask = 2 [
  (buf.validate.field).field_mask = {
    [pkg.mutable_paths]: ["display_name", "icon"]
  }
];

failed with the trait bound FieldMask: AsCelValue is not satisfied. Same shape for Timestamp- and Duration-typed fields.

Fix

Three new impl AsCelValue blocks in protovalidate-buffa/src/cel.rs:

• FieldMask -> CEL Map { paths: List<String> }, so this.paths.all(p, ...), size(this.paths), etc. work.
• Timestamp -> native CEL Timestamp via the existing timestamp_from_secs_nanos helper, so this < timestamp(...), this < now, int(this) work.
• Duration -> native CEL Duration via the existing duration_from_secs_nanos helper, so this > duration('0s') works.

Pulls buffa-types = "0.5" into the runtime crate to reference the WKT struct definitions. No changes to the codegen plugin or to public API beyond the new impls.

Test plan

• cargo test -p protovalidate-buffa — new tests/wkt_cel.rs exercises each impl through the real CelConstraint::eval path (this.paths.all(...), size(this.paths), this > duration('0s'), this < timestamp('9999-12-31T23:59:59Z'), this < now, sub-second nanos resolution).
• cargo test --workspace --all-targets — no regressions.
• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --all -- --check — clean.
• Downstream verification: bump consumer to released version, swap a message-level update_mask CEL for a field-level predefined rule on a google.protobuf.FieldMask field, confirm it compiles.