pw_bluetooth_sapphire: Fix shadow variable warnings

This fixes warnings about variable names being shadowed in cases such
as:
```
const auto result = foo();
if (result.is_error()) {
  auto result = bar();
  use_result(result);
}
```

Bug: b/355511476
Test: fx test //src/connectivity/bluetooth/core/bt-host

Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1089918
GitOrigin-RevId: e99becf4801c0346aa8cdae07362ecb0607edd43
Change-Id: I49eb2cc349728bc862468cfbfc2c5de0c345d817
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/227036
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Jason Graffius <jgraff@google.com>
Reviewed-by: Eric Rahm <erahm@google.com>