[milyin]

Applications can now write a sequence prefix with ext::Serializer::serialize_sequence_length() and read it with ext::Deserializer::deserialize_sequence_length(), then process elements individually. Decoders can enforce an application-defined limit before allocating a container.

Both methods process only the length prefix and follow the existing ZResult*/exception convention. With an error pointer, decoding failures return zero and set the result code; a successful empty sequence also returns zero with Z_OK. The serializer caller is responsible for writing the advertised number of elements. Existing container serializers and deserializers reuse these methods. The wire format is unchanged.

Tests cover manual sequence encoding compatible with container encoding, sequence elements and a following value, empty sequences, large length-only prefixes, and missing/truncated prefixes. Encoding is exercised with and without an error pointer; malformed prefixes are checked through both decoding error-reporting modes.

Validation

• The original decoding changes passed serialization CTests on macOS with the repository's pinned zenoh-c and zenoh-pico dependencies, with and without exceptions.
• The encoding addition passed serialization CTests on macOS for both backends, with and without -fno-exceptions, using cached Zetta dependencies. For zenoh-c, unstable APIs were disabled to avoid an unrelated Link API mismatch between the cached dependency and this revision.
• clang-format 18.1.3 and git diff --check passed.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves existing behavior and comprehensively tests the new API contract.

Review effort: Balanced
Findings: None

What changed in this PR

Adds safe sequence-length prefix decoding to the C++ deserialization API.

Changes:

• Exposes Deserializer::deserialize_sequence_length().
• Reuses it across container deserializers.
• Tests normal, empty, large, missing, and truncated prefixes.

File	Description
include/​zenoh/​api/​ext/​serialization.hxx	Adds the API and integrates it with container decoding.
tests/​universal/​serialization.cxx	Covers successful and failing prefix decoding.

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