[20257] Add a keyed fragmented change to the reader data instance only when its completed (backport #4261)

[mergify]

This is an automatic backport of pull request #4261 done by Mergify.
Cherry-pick of 9558ce4 has failed:

On branch mergify/bp/2.6.x/pr-4261
Your branch is up to date with 'origin/2.6.x'.

You are currently cherry-picking commit 9558ce436.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/cpp/rtps/reader/StatefulReader.cpp
	modified:   src/cpp/rtps/reader/StatelessReader.cpp
	modified:   test/blackbox/api/dds-pim/PubSubReader.hpp
	modified:   test/blackbox/api/fastrtps_deprecated/PubSubReader.hpp
	modified:   test/blackbox/common/BlackboxTestsPubSubFragments.cpp

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/cpp/fastdds/subscriber/history/DataReaderHistory.cpp

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[Mario-DL]

@richiprosima please test linux

[Mario-DL]

@richiprosima please test mac

[Mario-DL]

@richiprosima please test uncrustify

[Mario-DL]

@richiprosima please test discovery_server

[Mario-DL]

@richiprosima please test mac

[Mario-DL]

@richiprosima please test mac

[Mario-DL]

Rebasing its a pity as we would throw away all the job done in the already passed cis.
Asking a manual windows ci here

[Mario-DL]

Failed tests were fixed in #4275 , giving it as ready-to-merge