feat: [DSM-148] XNetClient::post_advert()

[alin-at-dfinity]

POSTs a header-only certified slice — our stream to the peer — and returns the peer's own certified header if it replied with one, i.e. if the advert brought it nothing new. A 204 is the ordinary outcome and not an error, unlike for query(), where it means there is no stream.

An advert above ADVERT_MAX_BODY_BYTES fails before being sent, as the receiver would refuse it and the fault is ours. Being refused by the peer (429, 403) leaves it healthy, as 4xx already does for query(); only the failures that say the node did not serve us mark it unhealthy.

Nothing sends adverts yet; the task that does, and that feeds a reply back through handle_advert(), follows.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 403ca7e.

Security Overview

• 🔎 Scanned files: 3 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=b613b398-299e-4b22-8311-56996728af6d&codeScanTypes=PrScan&tab=issues&detectionTypes=traditional,businessLogic,customRule,IAC,SECRETS,CICD,EOL,uncategorized

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Add XNetClient::post_advert API and implement its usage
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Introduce post_advert_impl, update_node_health, and health updates in XNetClientImpl
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Extend XNetClient trait to include post_advert method and adjust http client type to XNetRequestBody
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Handle advert lifecycle: size check, request/response flow, and status-based decoding
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Add AdvertTooLarge error variant and integrate into error handling and health logic
Enhancement	► rs/xnet/payload_builder/src/lib.rs     Refactor query path to use updated health tracking in XNetClientImpl
Enhancement	► rs/xnet/payload_builder/src/xnet_client_tests.rs     Add tests for advert path: advert_reply, advert_no_reply, advert_refused, advert_server_error, advert_too_large
Enhancement	► rs/xnet/payload_builder/src/xnet_client_tests.rs     Add test harness: do_xnet_client_advert, advert_endpoint helper, start_server, and get_stream_slice_for_testing usage
Enhancement	► rs/xnet/payload_builder/tests/xnet_payload_builder.rs     Provide mock implementation for post_advert to satisfy trait in tests