fix: retry manifest reads from URLs on transient network errors

[annaveretennykova]

Issue #: #4194

Description of changes:

Manifest reads from URLs in pkg/files/reader.go had no retry, so a single transient network error (for example a DNS i/o timeout) failed the whole operation. This wraps the HTTP GET and body read in readHttpFile with the existing pkg/retrier, so transient failures are retried instead of aborting the command. The Reader gets a retrier field defaulted to retrier.NewWithMaxRetries(5, 5*time.Second) in NewReader, plus a WithRetrier option for tests to inject a custom retrier. Only the network round trip is retried; request construction and local/embedded file reads are unchanged, and the existing error message is preserved.

This covers both failure paths in the issue (cluster upgrade and cluster delete), since both go through readHttpFile.

Testing:

Added two unit tests in pkg/files/reader_test.go using a TLS test server that closes the connection to simulate a transient error:

• retry then succeed on a later attempt, asserting the read succeeds and the expected number of attempts were made
• fail after exhausting the configured retries, asserting an error is returned after the expected number of attempts

go build ./pkg/files/..., go vet ./pkg/files/..., gofmt -l pkg/files/, and go test ./pkg/files/... all pass.

Documentation:

None. Internal behavior change with no user-facing configuration or API surface.

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