Fix CI failures: update doctest, add changelog, fix coverage

- Update extending.md doctest to reflect that field_transformer now
  receives pre-resolved aliases (use alias == name.lstrip("_") instead
  of `not field.alias` to detect auto-generated aliases)
- Add changelog entry for #1479
- Add test_hook_new_field_without_alias to cover the post-transformer
  alias resolution path (line 496 of _make.py)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: veeceey

changelog.d/1479.change

@@ -0,0 +1 @@
+Field aliases are now resolved *before* calling `field_transformer`, so transformers receive fully populated `Attribute` objects with usable `alias` values instead of `None`.

docs/extending.md

@@ -248,13 +248,13 @@ Data(a=3, b='spam', c=datetime.datetime(2020, 5, 4, 13, 37))
 ```
 
 Or, perhaps you would prefer to generate dataclass-compatible `__init__` signatures via a default field *alias*.
-Note, *field_transformer* operates on {class}`attrs.Attribute` instances before the default private-attribute handling is applied so explicit user-provided aliases can be detected.
+Note, *field_transformer* receives {class}`attrs.Attribute` instances with default aliases already resolved (e.g., leading-underscore stripping has been applied), so you can compare against the default to detect explicit user-provided aliases.
 
 ```{doctest}
 >>> def dataclass_names(cls, fields):
 ...     return [
 ...         field.evolve(alias=field.name)
-...         if not field.alias
+...         if field.alias == field.name.lstrip("_")
 ...         else field
 ...         for field in fields
 ...     ]

tests/test_hooks.py

@@ -292,6 +292,44 @@ class C:
 
         assert "my_alias" == attr.fields(C).renamed.alias
 
+    def test_hook_new_field_without_alias(self):
+        """
+        When a field_transformer adds a brand-new field without setting an
+        alias, the post-transformer alias resolution fills it in.
+
+        Regression test for #1479.
+        """
+
+        def hook(cls, attribs):
+            return list(attribs) + [
+                attr.Attribute(
+                    name="_extra",
+                    default=0,
+                    validator=None,
+                    repr=True,
+                    cmp=None,
+                    hash=None,
+                    init=True,
+                    metadata={},
+                    type=int,
+                    converter=None,
+                    kw_only=False,
+                    eq=True,
+                    eq_key=None,
+                    order=True,
+                    order_key=None,
+                    on_setattr=None,
+                    alias=None,
+                    inherited=False,
+                )
+            ]
+
+        @attr.s(auto_attribs=True, field_transformer=hook)
+        class C:
+            x: int
+
+        assert "extra" == attr.fields(C)._extra.alias
+
 
 class TestAsDictHook:
     def test_asdict(self):