fix(dream_skin): fix Windows-only compile error breaking Windows CI

[LeoLin990405]

Problem

main's Windows artifacts CI has been failing since 7385664 (last green) with a compile error:

error[E0599]: no method named `into_owned` found for struct `std::string::String`
  --> crates/codex-plus-core/src/dream_skin.rs:593:32

The #[cfg(windows)] normalized_identity_string builds raw as an owned String
(path.to_string_lossy().replace('/', "\\")), then calls raw.into_owned() in the final
unwrap_or_else. String has no into_owned() (that's Cow/Borrow), so it fails to
compile — but only on Windows, since the function is #[cfg(windows)] (non-Windows builds
use a separate branch, so macOS/Linux CI stays green and the break is easy to miss).

This blocks the Windows job for every PR rebased onto current main.

Fix

Return raw directly (it is already the owned String):

-        .unwrap_or_else(|| raw.into_owned());
+        .unwrap_or_else(|| raw);

The other into_owned() calls in the tree operate on Cow (to_string_lossy().into_owned())
and are fine; only this String case is wrong.

Verification

Since the function is #[cfg(windows)], verified with a Windows cross-target type-check
(rustc --target x86_64-pc-windows-msvc --emit=metadata on the isolated function):

• original || raw.into_owned() → reproduces error[E0599];
• fixed || raw → type-checks clean.

cargo check -p codex-plus-core on macOS stays green.