test: update format tests for segment-based output

Problem: format functions now return `forge.Segment[]` instead of
strings, so all tests calling `:find()` and `:match()` on results
were failing.

Solution: add `flatten()` helper that concatenates segment text, and
wrap all format assertions with it. Also fix stylua formatting in
`sources_spec.lua`.

spec/sources_spec.lua

@@ -86,7 +86,10 @@ describe('github', function()
   end)
 
   it('preserves in_progress status in normalize_run', function()
-    assert.equals('in_progress', gh:normalize_run({ databaseId = 1, status = 'in_progress' }).status)
+    assert.equals(
+      'in_progress',
+      gh:normalize_run({ databaseId = 1, status = 'in_progress' }).status
+    )
   end)
 end)