# DEVX-163: Fix _run_push to check stdout for HTTP 500

## Problem
`_run_push` only checked `result.stderr` for HTTP 500, but docker push
sends the "received unexpected HTTP status: 500 Internal Server Error"
message to **stdout**, not stderr. This means the tenacity retry logic
added in DEVX-162 never triggered — the push failed immediately without
retrying.

## Approach
Check both `result.stdout` and `result.stderr` for the "500" status code.
Also update the "already exists" check in `push_image` to check both
streams, since docker may send that message to stdout as well.

REQ-1: _run_push checks both stdout and stderr for HTTP 500
REQ-2: push_image "already exists" check uses combined stdout+stderr
REQ-3: All existing tests pass with 100% coverage

## Test Plan
- Unit tests for stdout 500 detection
- Unit tests for stderr 500 detection
- Manual: trigger build-images workflow and verify retry works

## Deploy Plan
- Merge to master

## Rollback Plan
- Revert the merge commit

## Acceptance Criteria
- [x] REQ-1: _run_push checks both stdout and stderr for HTTP 500
- [x] REQ-2: push_image "already exists" check uses combined stdout+stderr
- [x] REQ-3: All existing tests pass with 100% coverage