Fix marker merging for requirements.txt for psycopg (#11298)

Given an input in the shape:

```
foo[bar]==1.0.0; sys_platform == 'linux'
foo==1.0.0; sys_platform != 'linux'
```

We would write either

```
foo==1.0.0; sys_platform == 'linux'
```
or
```
foo==1.0.0
```

depending on the iteration order, as the first one is from the marker
proxy package and the second one from the package without marker.

The fix correctly merges graph entries when there are two nodes with
different extras and different markers.

I tried to write a packse test but it failed due to a different
iteration order showing the correct case directly instead of the failing
one we'd need.

Only `strip_extras` is affected, since `combine_extras` uses
`version_marker`.

crates/uv-resolver/src/resolution/display.rs

@@ -404,6 +404,13 @@ fn strip_extras<'dist>(graph: &IntermediatePetGraph<'dist>) -> RequirementsTxtGr
                 let index = *entry.get();
                 let node: &mut RequirementsTxtDist = &mut next[index];
                 node.extras.clear();
+                // Consider:
+                // ```
+                // foo[bar]==1.0.0; sys_platform == 'linux'
+                // foo==1.0.0; sys_platform != 'linux'
+                // ```
+                // In this case, we want to write `foo==1.0.0; sys_platform == 'linux' or sys_platform == 'windows'`
+                node.markers.or(dist.markers);
             }
             std::collections::hash_map::Entry::Vacant(entry) => {
                 let index = next.add_node(dist.clone());