Fix env name with dot losing description in TOML config (#3722)

## Description

Fixes #3590

Environment names containing dots (e.g. `"py3.11"`) in `pyproject.toml`
had their descriptions silently ignored. `tox list` showed `[no
description]` instead of the configured description.

### Reproducer

```toml
[tool.tox.env."py3.11"]
description = "tox test"
```

```console
$ tox list
py3.11 -> [no description]
```

### Root cause

Three interrelated issues in `toml_pyproject.py`:

1. **`sections()` split the env name on dots**: `from_key("py3.11")`
treats `.` as the section separator, creating `Section(prefix="py3",
name="11")` instead of preserving `"py3.11"` as the name.

2. **`keys` property split ALL dots**: The full key
`"tool.tox.env.py3.11"` was split into `["env", "py3", "11"]`.
`get_loader()` then tried to traverse `dict["py3"]["11"]` instead of
`dict["py3.11"]`, returning `None` (config not found).

3. **`envs()` yielded full key**: When `sections()` is fixed to use
`test_env()`, the full key becomes `"tool.tox.env.py3.11"` instead of
just `"py3.11"`, causing a phantom duplicate environment.

### Fix

- **`keys` property**: Build from `prefix` and `name` components
directly instead of splitting the joined key on `SEP`. This preserves
dots within the env name while still splitting structural path
separators.
- **`sections()`**: Use `test_env(env_name)` which correctly sets
`prefix="tool.tox.env"` and `name="py3.11"` as an atomic unit.
- **`envs()`**: Yield `section.name` instead of `section.key` to return
just the env name.
- **`get_base_sections()`**: Use `test_env()` for consistency.

### After fix

```console
$ tox list
py3.11 -> tox test
```

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

docs/changelog/3590.bugfix.rst

@@ -0,0 +1 @@
+Fix env names containing dots (e.g. ``py3.11``) losing their description in TOML configuration - by :user:`Fridayai700`.

src/tox/config/source/toml_pyproject.py

@@ -55,11 +55,26 @@ def run_env_base(cls) -> str:
 
     @property
     def keys(self) -> Iterable[str]:
-        key = self.key
-        keys = key.split(self.SEP) if self.key else []
-        if self.PREFIX and len(keys) >= len(self.PREFIX) and tuple(keys[: len(self.PREFIX)]) == self.PREFIX:
-            keys = keys[len(self.PREFIX) :]
-        return keys
+        # Build keys from prefix and name components directly, rather than
+        # splitting the joined key on SEP.  This preserves dots that are part
+        # of the name (e.g. environment name "py3.11") instead of treating
+        # them as path separators.
+        prefix = self._prefix
+        name = self._name
+        if prefix is None and not name:
+            return []
+        prefix_parts: list[str] = prefix.split(self.SEP) if prefix else []
+        # Strip the global PREFIX (e.g. ("tool", "tox")) from the front
+        if (
+            self.PREFIX
+            and len(prefix_parts) >= len(self.PREFIX)
+            and tuple(prefix_parts[: len(self.PREFIX)]) == self.PREFIX
+        ):
+            prefix_parts = prefix_parts[len(self.PREFIX) :]
+        result = prefix_parts
+        if name:
+            result.append(name)
+        return result
 
 
 class TomlPyProjectSection(TomlSection):
@@ -113,17 +128,17 @@ def get_loader(self, section: Section, override_map: OverrideMap) -> Loader[Any]
 
     def envs(self, core_conf: CoreConfigSet) -> Iterator[str]:
         yield from core_conf["env_list"]
-        yield from [i.key for i in self.sections()]
+        yield from [i.name for i in self.sections()]
 
     def sections(self) -> Iterator[Section]:
         for env_name in self._our_content.get(self._Section.ENV, {}):
             if not isinstance(env_name, str):
                 msg = f"Environment key must be string, got {env_name!r}"
                 raise HandledError(msg)
-            yield self._Section.from_key(env_name)
+            yield self._Section.test_env(env_name)
 
     def get_base_sections(self, base: list[str], in_section: Section) -> Iterator[Section]:  # noqa: ARG002
-        yield from [self._Section.from_key(b) for b in base]
+        yield from [self._Section.test_env(b) for b in base]
 
     def get_tox_env_section(self, item: str) -> tuple[Section, list[str], list[str]]:
         return self._Section.test_env(item), [self._Section.run_env_base()], [self._Section.package_env_base()]