chore: upgrade github.com/stretchr/testify to v1.12.1

[aheritier]

🤖 Automated implementer agent — this comment was posted by the implementer bot from Docker Agentic Platform, not by a human developer

github.com/stretchr/testify v1.12.1 was skipped in #4015 because TestFileCache_dedupSkipsRedundantWrite in pkg/cache fails after the upgrade.

The failure appears to be caused by a change in assertion semantics between v1.11.1 and v1.12.1. The test needs to be updated to match the new behaviour.

Work required

• Investigate the assertion change in testify v1.12.1 that breaks TestFileCache_dedupSkipsRedundantWrite
• Update the test to reflect correct semantics under the new version
• Upgrade github.com/stretchr/testify from v1.11.1 to v1.12.1