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/util.py

@@ -122,6 +122,8 @@ async def run_askpass(
                         continue  # Can't get user name from SSH_ASKPASS
                     case AskpassKey.Password:
                         exe_arg += 'Password'
+            case _:
+                pass
         result = await run_cmd([exe, exe_arg], throw = throw, ec = ec)
         if result.status == 0 and result.stdout_or_none is not None:
             ret = result.stdout_str_or_none