Preserve native trailing command-line argument bytes - #9494

Summary

• Preserve native trailing command-line arguments as OsString instead of rejecting them during CLI UTF-8 conversion.
• Decode sys.argv and sys.orig_argv through the existing filesystem-decoding path; use args_os() for original arguments and program-name initialization.
• Adapt startup error reporting and the syslog default identifier to the retained native strings.
• Restore only CmdLineTest.test_invalid_utf8_arg through scripts/update_lib migrate, retaining every other failure marker and the explicit RUSTPYTHONPATH adapter. No test assertion or payload changes.

Based on main 8051c05ae4e100bf35eeb32044d3456d2546dc75, after #9492 landed. No dependency or Cargo.lock changes.

Public Rust API change

Settings.argv changes from Vec<String> to Vec<OsString> and Paths.program_name from String to OsString. Embedders assigning these public fields must convert their values (for example with .into() or OsString::from). This is not a source-compatible field-type change.

Scope and limits

This fixes native-byte transport for trailing arguments and original-argument construction under RustPython's existing filesystem-decoding policy. It does not implement complete locale-dependent decoding, non-Unicode script/module selectors, invalid -c diagnostics, or invalid argv[0] compatibility. Those behaviors are not claimed by this PR. The undecodable-code expected-failure marker remains.

Validation

• cargo fmt --all -- --check and git diff --check passed.
• cargo check and strict cargo clippy -- -D warnings passed for --bin rustpython --no-default-features --features threading,stdlib,stdio,importlib,host_env,doc,ucd_3_2_0 with the unchanged lockfile.
• Fresh CLI build passed with the same features; the actual VM compiler command used opt-level 3.
• Fresh bounded observations compared the exact two canonical test_invalid_utf8_arg payloads under C.UTF-8/default, C locale, -X utf8, and -X utf8=0: eight candidate and eight pinned CPython 3.15.0 processes. Every process exited successfully without stderr; os.fsencode(sys.argv[1]) and os.fsencode(sys.orig_argv[-1]) matched the original bytes and the reference. Input manifests remained unchanged and all owned process groups exited.
• The canonical test method's AST, including its remaining platform decorator, matches CPython commit d220409a13b77de7a6c30abe16ea90db01528edc.

The observations directly ran the bounded payload observer with -B -S; they did not execute the original subprocess unittest harness, the entire test_cmd_line module, the full workspace/CAPI suites, or Windows/macOS runtime tests. The reference was built with test extension modules disabled. CI remains the next cross-platform check, not proof of untested compatibility.

AI-assisted implementation and validation; commit disclosure included.