[8.19](backport #7007) Fix missing authAgent in UploadChunk

[mergify]

What is the problem this PR solves?

In the UploadChunk the authAgent was not being called to validate that the uploading agent of the chunk has the permissions to write that chunk. It validated that there was a valid API key, it just didn't validate that the agent with the valid API key is the agent that should be uploading the chunk.

In ValidAPIKey a disabled API key was considered valid even once disabled.

How does this PR solve the problem?

It adds the authAgent check to the UploadChunk.

It sets ok = false when the API key is disabled in CacheT.ValidAPIKey.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• [ ] I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

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This is an automatic backport of pull request #7007 done by [Mergify](https://mergify.com).