Preserve concurrent spec edits during Redis status updates

[fkautz]

Upstream PR creation is restricted, so this review PR is hosted in the fork. Its base matches google/ax main.

UpdateTaskStatus currently overwrites an entire stale task snapshot. Protect the read and write with WATCH/MULTI and bounded retries so a concurrent spec edit survives. Status publication occurs only with a successful transaction. Normalize an omitted atespace before selecting the key.

Fixes google#352.

TDD and validation

This branch starts independently from origin/main (d8ed0fe) and has two commits:

1. 281c488 adds the regression and fails on the original behavior.
2. 3ad3d2d implements the fix; the regression and complete suite pass.

Reproduce the red/green transition by checking out either commit and running:

go test ./internal/store/redis -run '^TestReviewStatusUpdatePreservesConcurrentSpecEdit$' -count=1 -v

Validated: go test ./... -count=1, targeted -race tests, go mod tidy with unchanged module files, make build, and git diff --check. Redis tests launch isolated real servers; redis-server is installed in CI and documented for local development.

Review

Codex review: no actionable regressions identified. Some reviewer test runs were blocked by its listener sandbox; the validation above was run separately with local listener access.