Fix Age header value type to string for PSR compliance

[ievgeniiskliarenko]

Problem

guzzlehttp/psr7 2.11.0 deprecated passing non-string values to MessageInterface::withHeader(),
emitting E_USER_DEPRECATED via trigger_deprecation().

Stacktrace:
MessageTrait.php: trigger_deprecation('guzzlehttp/psr7', '2.11', 'Passing %s to M...', 'int')
CacheEntry.php: Response->withHeader('Age', 0) ← int, not string

Fix

Cast getAge() return value to string before passing to withHeader().

Compatibility

• Fixes psr7 2.11+ deprecation
• Fully backwards compatible with older psr7 versions

[gemini-code-assist]

Code Review

This pull request casts the 'Age' header value to a string in CacheEntry.php and adds a test to verify this behavior. The review feedback points out a PSR-12 formatting issue regarding the space after the casting operator, and notes that the test assertion is redundant because getHeaderLine() always returns a string; it suggests asserting on the array returned by getHeader() instead.

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[Kevinrob]

Thanks @ievgeniiskliarenko !

[Kevinrob]

Available in 8.0.0-RC2