lib, cmds: Add exhaustive match cases

Add 'case _: pass' to match statements that are intentionally
non-exhaustive, satisfying the new exhaustive-match mypy rule.

Also replaced 'case '_':' (a string literal) with 'case _: pass' in
pkg_relations.py since it was an unreachable case (syntax is a
VersionSyntax enum, not a str).

Added 'case VersionSyntax.names_only:' to the match in pkg_relations.py
to handle the missing enum value.

Files modified:
- util.py: Two match statements for askpass env vars
- Distro.py: Three match statements for backend/os detection
- pkg_relations.py: Match on VersionSyntax enum
- CmdListRepos.py: Match on URL scheme

Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev 0.81.1
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/Distro.py

@@ -98,6 +98,8 @@ class Distro(abc.ABC):
         match ret:
             case 'opensuse-tumbleweed':
                 return 'suse'
+            case _:
+                pass
         return ret
 
     @classmethod
@@ -119,6 +121,8 @@ class Distro(abc.ABC):
                 backend_id = 'redhat'
             case 'opensuse' | 'suse':
                 backend_id = 'suse'
+            case _:
+                pass
         module_path = 'jw.pkg.lib.distros.' + backend_id + '.Distro'
         try:
             module = importlib.import_module(module_path)
@@ -173,6 +177,8 @@ class Distro(abc.ABC):
                 __append('linux')
                 __append('pkg-pm')
                 __append('pm-pacman')
+            case _:
+                pass
 
         os = self.os
         name = re.sub(r'-.*', '', os)