Fix volatile reader desync after initial positioning GAP

[cferreiragonz]

Description

This PR is an alternative to fix proposed for CVE-2025-64438 that limited the maximum number of GAPs that a reader could process. This fix had a problem with volatile late-joiners readers that caused a de-synchronization between reader and writer, resulting in the reader not processing any message. The key is that the writer assumes the reader's state after sending the first initial positioning GAP in StatefulWriter::matched_reader_add_edp. Hence, later ACKs requesting old data are simply ignored and the reader never syncs with the writer, preventing real communication even though DATA is correctly being received at the transport layer.

The issue comes, thus, from two different conditions:

• StatefulReader not processing all GAPs sent by the writer (similar behavior will happen if the GAP message is lost or sent before the reader match the writer on its side - Discovery race condition)
• StatefulWriter assuming the reader's state after sending a positioning GAP

The new fix forces sending a HB before every GAP and limiting the GAP processing to the value specified in the heartbeat. It also allows the Writer to send GAP messages if a later ACK is received requesting old samples.

@Mergifyio backport 3.2.x 2.14.x

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• Any new/modified methods have been properly documented using Doxygen.
• N/A: Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• N/A: New feature has been added to the versions.md file (if applicable).
• N/A: New feature has been documented/Current behavior is correctly described in the documentation.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.