Add replication destination prefix support

[mother-6000]

Closes #134

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• CHANGELOG.md has been updated (for bug fixes / features / docs)

What kind of change does this PR introduce?

Feature.

What was changed?

• Added IClient::ReplicationSettings::dst_prefix for destination entry prefix configuration.
• Serialized non-empty dst_prefix in replication create/update payloads while omitting the default empty value for backward compatibility.
• Parsed returned settings.dst_prefix from full replication details, defaulting to an empty string for older responses.
• Added serialization/parser coverage and a live create/update/get round-trip test tagged [1_20].
• Wrapped an existing long token API assertion so the repository cpplint command passes on current main.

Related issues

• Issue: C++ SDK: add replication prefix setting #134
• Plan: C++ SDK: add replication prefix setting #134 (comment)
• Core implementation: add replication entry prefix reductstore#1486

Does this PR introduce a breaking change?

No. The public settings struct change is additive, empty dst_prefix is omitted from request payloads, and older server responses without the field still deserialize with an empty prefix.

Other information:

Local validation:

• .venv/bin/cpplint over src/, tests/, and examples/
• cmake -S . -B build -DREDUCT_CPP_USE_FETCHCONTENT=ON -DREDUCT_CPP_ENABLE_TESTS=ON
• cmake --build build
• REDUCT_CPP_TOKEN_API=TOKEN ./build/bin/reduct-tests against reduct/store:main
• REDUCT_CPP_TOKEN_API=TOKEN ./build/bin/reduct-tests "~[1_20]" against reduct/store:latest