RTECO-1263 - Fixed Content Reader race panic

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Summary

Fixes panic: send on closed channel in ContentReader triggered intermittently by concurrent use of NextRecord and Reset (or by callers that invoke Reset internally, e.g.
Length, SortContentReader).

Resolves RTECO-1263.

Background

ContentReader.NextRecord spawns a producer goroutine once (via sync.Once) that streams JSON array elements from one or more files into cr.dataChannel. The receiver in
NextRecord and the producer in readSingleFile both read cr.dataChannel from the struct field on each access.

Reset() replaces both cr.dataChannel and cr.once without any synchronization. This created three independent race conditions that together produce the panic:

1. Producer / Reset race — a producer goroutine paused between sends (between files, or while blocked on a full buffer) could resume after Reset() had swapped the channel
   field. The producer's next send would land on the new channel, which could already have been closed by the next cycle's defer close.
2. Receiver / Reset race — NextRecord could fire once.Do against the old once, then read cr.dataChannel after Reset() had swapped both fields, causing the receiver
   and producer to operate on different channels (deadlock).
3. Defer / send mismatch — defer close(cr.dataChannel) evaluates the channel argument at defer-registration time, but cr.dataChannel <- item re-reads the field on every
   send. The defer and the send could refer to two different channels.

The race is timing-dependent; it surfaces in CI under load (e.g. TestPushFatManifestImageWithNestedPath in the docker test suite) but is hard to reproduce locally without -race
and concurrent stress.

Stack trace from the failing CI run:

panic: send on closed channel
goroutine 2699 [running]:
github.com/jfrog/jfrog-client-go/utils/io/content.(*ContentReader).readSingleFile
.../utils/io/content/contentreader.go:216 +0x492
github.com/jfrog/jfrog-client-go/utils/io/content.(*ContentReader).run(...)
.../utils/io/content/contentreader.go:178
github.com/jfrog/jfrog-client-go/utils/io/content.(*ContentReader).NextRecord.func1.1()
.../utils/io/content/contentreader.go:78 +0x7a