VisibleTimeRange is now a union, add serialization comparision test (…

…#6259)

### What

Part of a train 
* #6257
* #6258
* ➡️ #6259

Replaces a fair chunk of
* #6221

Makes `VisibleTimeRange` a union - the motivator for this pr train in
the first place. Adds a roundtrip test that compares the serialization
of`VisibleTimeRange` archetype across sdk languages. (the archetype is
meant for blueprint only, but I'm simply using the datastore for this
comparision)

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6259?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/6259?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/6259)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

crates/re_types/definitions/rerun/blueprint.fbs

@@ -27,7 +27,7 @@ include "./blueprint/archetypes/panel_blueprint.fbs";
 include "./blueprint/archetypes/space_view_blueprint.fbs";
 include "./blueprint/archetypes/space_view_contents.fbs";
 include "./blueprint/archetypes/viewport_blueprint.fbs";
-include "./blueprint/archetypes/visible_time_range.fbs";
+include "./blueprint/archetypes/visible_time_ranges.fbs";
 include "./blueprint/archetypes/visual_bounds.fbs";
 
 include "./blueprint/archetypes/plot_legend.fbs";

crates/re_types/definitions/rerun/datatypes/visible_time_range.fbs

@@ -4,35 +4,25 @@ include "python/attributes.fbs";
 include "rust/attributes.fbs";
 
 include "rerun/attributes.fbs";
+include "rerun/unit_type.fbs";
 
 include "time_int.fbs";
 
 namespace rerun.datatypes;
 
-/// Kind of boundary for visible history, see `TimeRangeBoundary`.
-enum TimeRangeBoundaryKind: byte (
+/// Left or right boundary of a time range.
+union TimeRangeBoundary (
+  "attr.rust.derive": "Copy, PartialEq, Eq",
   "attr.rust.override_crate": "re_types_core"
 ) {
   /// Boundary is a value relative to the time cursor.
-  RelativeToTimeCursor,
+  CursorRelative: TimeInt,
 
   /// Boundary is an absolute value.
-  Absolute,
+  Absolute: TimeInt,
 
   /// The boundary extends to infinity.
-  Infinite,
-}
-
-/// Type of boundary for visible history.
-struct TimeRangeBoundary (
-  "attr.rust.derive": "Copy",
-  "attr.rust.override_crate": "re_types_core"
-) {
-  /// Type of the boundary.
-  kind: rerun.datatypes.TimeRangeBoundaryKind (order: 100);
-
-  /// Value of the boundary (ignored for `Infinite` type).
-  time: rerun.datatypes.TimeInt (order: 200);
+  Infinite: rerun.builtins.UnitType,
 }
 
 /// Visible time range bounds for a specific timeline.

crates/re_types_builder/src/codegen/python/mod.rs

@@ -1219,7 +1219,7 @@ fn quote_union_kind_from_fields(fields: &Vec<ObjectField>) -> String {
                 line.remove(0);
             }
         }
-        lines.push(format!("* {:?}:", field.name));
+        lines.push(format!("* {:?}:", field.snake_case_name()));
         lines.extend(content.into_iter().map(|line| format!("    {line}")));
         lines.push(String::new());
     }