net/unicoap: initialize .payload_representation in message_init helpers

[adityachilka1]

Contribution description

Fixes #22285.

The four unicoap_message_init* inline initializers in sys/include/net/unicoap/message.h populated code, options, payload, and payload_size but never wrote the .payload_representation : 1 bitfield. The function-level contract above each initializer explicitly tells callers they do not need to zero the unicoap_message_t struct beforehand — but with a stack-allocated un-zeroed struct, the representation bit ends up holding whatever was on the stack. When that bit happens to be 1 (UNICOAP_PAYLOAD_NONCONTIGUOUS), subsequent calls to unicoap_message_payload_get_size, the chunks accessors, or any serialisation routine enter the noncontiguous codepath and dereference message->payload_chunks as an iolist_t* — even though the caller populated the contiguous-payload fields.

This PR adds message->payload_representation = UNICOAP_PAYLOAD_CONTIGUOUS; to each of the four initializers (unicoap_message_init_empty, unicoap_message_init, unicoap_message_init_string, unicoap_message_init_with_options).

The sibling unicoap_message_alloc_* helpers already get correct behaviour for free via designated initializers (any field not mentioned is value-initialised to zero, which is UNICOAP_PAYLOAD_CONTIGUOUS).

The fifth unicoap_message_init_string_with_options is deliberately unchanged — it delegates to unicoap_message_init_with_options internally, so it inherits the fix transitively.

Testing procedure

Before this patch, the following reproducer (taken from the issue body) can segfault depending on what happened to be on the stack:

unicoap_message_t message;  /* deliberately not zero-initialised */
unicoap_message_init_string(&message, code, "hello");
size_t n = unicoap_message_payload_get_size(&message);  /* may take noncontiguous branch and crash */

After the patch, message.payload_representation is always UNICOAP_PAYLOAD_CONTIGUOUS after any unicoap_message_init* call, so the contiguous-payload branch in unicoap_message_payload_get_size is always taken. The asserts in unicoap_message_payload_get and unicoap_message_payload_get_chunks keep the contract honest.

You can exercise this in the existing unicoap example app (examples/networking/coap/unicoap):

make -C examples/networking/coap/unicoap all
make -C examples/networking/coap/unicoap test  # if a test target exists for this board

The change is header-only and additive, so any build that pulls in net/unicoap/message.h covers it.

Issues/PRs references

Fixes #22285.

Declaration of AI-Tools / LLMs usage

AI-Tools / LLMs that were used are:

• Claude (Sonnet) for code analysis, identifying the four direct-write initializers vs. the fifth delegating one, drafting the commit message and this PR body, and confirming the constant name (UNICOAP_PAYLOAD_CONTIGUOUS) by reading the existing setter unicoap_message_payload_set and the asserts in the contiguous/noncontiguous getters — with user (@adityachilka1) review of every code line before submission ("agent mode" with explicit user review at each step).

The C source change itself is mechanical (one identical line added to four already-existing function bodies) and matches the exact pattern used by the unicoap_message_payload_set setter elsewhere in the same file. No new logic is introduced.

[github-actions]

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