fix(agent): gate the cache test helper like its only caller

[LutzLange]

Fixes #153.

CredentialCache::get is only called from the llm_proxy test module, which is gated to Linux. On macOS and Windows the helper was compiled without a caller, so make check (clippy -D warnings) failed with method get is never used. The helper now carries the same cfg(all(test, target_os = "linux")) as the module, as @jagansanikommu suggested in the issue (thanks for the analysis).

Checked on Linux: cargo fmt --check, cargo clippy -p agentdesktop-agent --all-targets -- -D warnings, and the 41 llm_proxy tests pass. For the non-Linux case I turned the test module's gate off on Linux: before the change clippy reports the error from the issue, after it clippy passes. Not run on a real macOS or Windows host.

🤖 Generated with Claude Code

[jagansanikommu]

Verified this on a real macOS host, since that was the gap you flagged.

macOS 27.0, arm64, Rust 1.98 (the pinned toolchain).

On main at 0a1bb78, before your change:

error: method `get` is never used
  --> crates/agent/src/llm_proxy.rs:149:8
error: could not compile `agentdesktop-agent` (lib test) due to 1 previous error

On this branch:

cargo clippy -p agentdesktop-agent --all-targets -- -D warnings   clean
cargo clippy --workspace --all-targets -- -D warnings             clean
cargo test -p agentdesktop-agent --lib                            333 passed

So the real-host case matches what your Linux simulation predicted, and make check passes on macOS with this applied. I have no Windows machine, so that half is still unverified — though it should follow, since the cause is the target_os gate rather than anything platform-specific beyond it.

Thanks for picking it up, and for the credit.