Refactor message marshalling - parent path max length

[werwurm]

The maximum path length is no longer hard coded/set at compile time. Instead, parsing the parent path is deferred, so that the number of compressed input slices need not be known and allocated in advance.

[github-actions]

LCOV of commit 9ed4bc7 during lcov-test-coverage-report #116

Summary coverage rate:
  lines......: 95.4% (2733 of 2864 lines)
  functions..: 99.1% (209 of 211 functions)
  branches...: 86.3% (1495 of 1733 branches)

Files changed coverage rate: n/a

[Copilot]

Pull request overview

Refactors NAT20 service message marshalling for the “parent path” field to remove the compile-time maximum path length and defer decoding until iteration, reducing up-front allocation requirements.

Changes:

• Introduces n20_parent_path_t (encoded/decoded representation) plus n20_msg_parent_path_iterate() for on-demand traversal.
• Updates message read/write logic to preserve an encoded CBOR slice for parent paths instead of decoding into a fixed-size array.
• Refactors/extends unit tests to cover round-trips and iterator behavior for both encoded and decoded parent-path inputs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
include/nat20/service/messages.h	Adds n20_parent_path_t API + iterator and updates request payload structs to use it.
src/service/messages.c	Implements deferred parent-path parsing, iterator, and updates request read/write paths accordingly.
src/service/test/messages.cpp	Updates round-trip tests and adds iterator success/error-path tests for parent paths.

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[Copilot]

Pull request overview

This PR refactors NAT20 service message marshalling to remove the compile-time maximum parent-path length by storing parent paths as either an encoded CBOR slice or a decoded slice array, and deferring per-element parsing until iteration.

Changes:

• Replace fixed-size parent_path arrays + parent_path_length with n20_parent_path_t across request payloads.
• Update message read/write logic to preserve an encoded parent-path slice and add n20_msg_parent_path_iterate() for on-demand decoding.
• Refactor/extend tests to validate round-trips and parent-path iteration behavior (including error propagation).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 8 comments.

File	Description
src/service/messages.c	Implements encoded/decoded parent-path read/write, adds iterator API, and updates request parsing/serialization.
include/nat20/service/messages.h	Introduces the n20_parent_path_t public type + iterator API and updates request payload structs to use it.
src/service/test/messages.cpp	Updates round-trip tests to use n20_parent_path_t and adds iterator success/error tests.

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[seidelrj]

nit: remove extra newline.

Suggested change