core: make test-log a dev dependency

test-log is only used by codex-core tests, so it does not need to be part of
the normal codex-core dependency graph. Keeping it in dev-dependencies removes
it from normal builds and keeps the production dependency set a little smaller.

Verification:

- `cargo tree -p codex-core --edges normal --invert test-log`
- `cargo check -p codex-core --lib`
- `cargo test -p codex-core --lib`

codex-rs/core/Cargo.toml

@@ -104,7 +104,6 @@ sha2 = { workspace = true }
 shlex = { workspace = true }
 similar = { workspace = true }
 tempfile = { workspace = true }
-test-log = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true, features = [
     "io-std",
@@ -166,6 +165,7 @@ opentelemetry_sdk = { workspace = true, features = [
 ] }
 serial_test = { workspace = true }
 tempfile = { workspace = true }
+test-log = { workspace = true }
 tracing-opentelemetry = { workspace = true }
 tracing-subscriber = { workspace = true }
 tracing-test = { workspace = true, features = ["no-env-filter"] }