BES upload failures override build status exit codes, making build success/failure detection unreliable

## Description
When a BES (Build Event Service) upload fails, Bazel changes its exit code to `PERSISTENT_BUILD_EVENT_SERVICE_UPLOAD_ERROR` regardless of whether the original build succeeded or failed. This behavior makes it impossible to programmatically determine the actual build status when a BES upload issue occurs.

## Impact
This issue impacts CI/CD pipelines and tools that need to reliably determine if a build passed or failed. Currently, when a BES upload fails, these systems cannot distinguish between:
- A successful build with a BES upload failure
- A failed build with a BES upload failure

## Expected Behavior
Bazel should expose some flag which allows the user to differentiate between these two cases. Currently a CI system would need to consume the BES to make such a distinction which is a high bar. Event then the only BES property which might help differentiate between exit codes is BuildFinished['overall_success'] which is considered deprecated see https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/buildeventstream/proto/build_event_stream.proto#L885

## Previous Workaround
Prior to the removal of `--bes_upload_mode=best_effort`, this issue could be worked around by setting that flag, which would cause Bazel to report BES upload failures but still exit with the original build status code. Without this option, there is no reliable way to distinguish build failures from upload failures.

## Additional Context
The underlying issue is that `PERSISTENT_BUILD_EVENT_SERVICE_UPLOAD_ERROR` takes precedence over `BUILD_OR_PARSING_FAILURE` in the exit code determination, whereas our systems had assumed the opposite precedence.

## Possible Solutions
1. Restore the `--bes_upload_mode=best_effort` option
2. Add a new flag targeted for CI systems which alters exit code precedence, making exit code 1 take precedence over all others. Then any non 1 exit code could result in a passing buildhe

Start by reviewing the exit-code precedence described in the issue and the referenced src/main/java/com/google/devtools/build/lib/buildeventstream/proto/build_event_stream.proto definition, including BuildFinished['overall_success']. Compare the proposed flag options and confirm that the resulting behavior distinguishes a successful or failed build from a BES upload failure.

Written by the indexing model from the issue text.