[extension/opamp] avoid host ID dependency in OS description

[jeevan6996]

Summary

• switch OS description lookup from host.Info() to host.PlatformInformation()
• avoid relying on host ID retrieval while composing the OpAMP agent OS description
• keep fallback behavior to runtime GOOS if platform lookup fails
• add tests that verify fallback on lookup failure and expected description formatting

Fixes #46965.

Testing

• cd extension/opampextension && go test ./...

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