fix: skip clock skew calculation for cached responses (Age header)

[toller892]

Problem

When a response is served from a cache (e.g. CloudFront), the Date header reflects when the original response was generated, not when it is being served. The SDK's RecordResponseTiming middleware uses this Date header to compute clock skew, which produces incorrect results for cached responses.

Once skew is calculated incorrectly, the next AWS call fails with a signature expiration error. The failing response then has a correct Date header, so subsequent calls succeed — but every first call after a cached response fails.

See #3409 for full reproduction steps and raw HTTP traces.

Fix

Check for the standard HTTP Age header in RecordResponseTiming.HandleDeserialize. When present, skip clock skew computation entirely, as the Date header is unreliable for skew calculation on cached responses.

This approach was suggested by @lucix-aws in #3409:

We will likely just specify to check for an Age header and if so disregard any potential skew for that request.

Changes

• aws/middleware/middleware.go: Add Age header check before computing skew
• aws/middleware/middleware_test.go: Add test case for cached response with Age header

Fixes #3409