fix(str): import alloc::borrow::ToOwned in no_std + alloc builds

[Greg Lamberson (glamberson)]

Summary

Closes the no_std + alloc compile failure in ironrdp-str flagged as
Category A of the workspace feature-matrix scan in #1123
(#1123 (comment)).

Approach

ironrdp-str/src/prefixed.rs and ironrdp-str/src/unframed.rs both
define impl From<&str> blocks that call s.to_owned() to lift the
borrowed string into an owned representation. .to_owned() is a method
from the alloc::borrow::ToOwned trait. In std builds the std prelude
brings ToOwned into scope automatically; the alloc prelude does not.
With --no-default-features --features alloc the call sites fail with
E0599: no method named 'to_owned' found for reference '&str' in the current scope.

Adds:

#[cfg(not(feature = "std"))]
use alloc::borrow::ToOwned;

to both files, matching the existing pattern that gates the
use alloc::{string::String, vec::Vec}; imports on the same cfg.
lib.rs already gates these modules behind #[cfg(feature = "alloc")],
so alloc is guaranteed available wherever the import compiles.

Verification

• cargo check -p ironrdp-str — green
• cargo check -p ironrdp-str --features std — green
• cargo check -p ironrdp-str --no-default-features --features alloc — green (was failing before)
• cargo check -p ironrdp-str --no-default-features — green
• cargo xtask check fmt -v — green
• cargo xtask check lints -v — green
• cargo xtask check tests -v — green
• cargo xtask check typos -v — green

References

• CI: compile/test a feature matrix including arbitrary (std/no_std) #1123 Category A: CI: compile/test a feature matrix including arbitrary (std/no_std) #1123 (comment)

[Benoît Cortier (CBenoit)]

Thanks! LGTM!