test: cover multi-block system preservation across the Messages gateway loop

[ashwing]

Summary

Adds a regression guard that a multi-block Anthropic system field survives the /v1/messages gateway tool loop unchanged across rounds.

The loop rebuilds the upstream request body every round — append_round_to_history appends the assistant turn + tool_result to messages. It only touches messages, so system should ride through untouched, but the loop path had no coverage of that (only the transparent-proxy path asserted system forwarding). Claude Code sends system as an attribution block + instructions, so this is the shape that matters.

The test drives a two-round gateway loop with a multi-block system and asserts both upstream request bodies carry the identical blocks — so a future refactor that rebuilds the upstream body from a curated field subset can't silently drop system.

Completes the Stage 3 (#116) system-attribution parity item. Part of #113.

Test Plan

• cargo test -p agentic-server-core --test messages_loop_test --test messages_stream_test (15 tests, including non-streaming and streaming multi-block system preservation)
• cargo fmt -- --check
• cargo clippy --all-targets -- -D warnings

[franciscojavierarceo]

The new regression guard covers only the non-streaming run_messages_loop path. Claude Code sends stream: true, and /v1/messages routes those requests through the independent run_messages_stream implementation and its own round-history mutation. Without a matching request-body assertion in messages_stream_test.rs, a streaming-only refactor could still drop or reshape the multi-block system field while this test remains green.

I’m pushing the corresponding streaming request-capture regression test to this branch before merge.

[franciscojavierarceo]

Verified the non-streaming and streaming multi-block system regression coverage, including a mutation check against the streaming path. Local cargo test, formatting, and Clippy all pass.