fix: reject incomplete Messages tool-loop streams

[franciscojavierarceo]

Summary

Require an upstream message_stop before a Messages built-in tool round can dispatch calls or finish successfully. Previously, EOF after stop_reason: tool_use could execute a tool, and truncated text rounds received a synthetic successful completion. The existing accumulator now emits an Anthropic SSE api_error without terminal message_delta or message_stop when the upstream ends early, including via [DONE].

Part of #313 and Enterprise Readiness #316. This is the completion gate only: full content-block lifecycle validation, aggregate retained-data budgets, and bounded tool scheduling remain open. Valid message_stop still ends a round without waiting for HTTP EOF; named-tool compatibility, pass-through requests, and upstream error forwarding retain their behavior.

Test Plan

• Reproduced the premature-success failure with the new regression test on the original implementation.
• Core library: 753 passed, 5 ignored. Messages stream integration suite: 8 passed, including ten incomplete-stream scenarios covering text/tool rounds, absent/present terminal deltas, EOF/[DONE], empty bodies, and an incomplete round after successful tool execution. Assertions count actual search calls and upstream requests.
• HTTP Messages completion suite: 12 passed, including terminal-before-EOF behavior and fragmented UTF-8. HTTP tool-choice suite: 13 passed.
• Workspace all-target clippy with warnings denied, all-files pre-commit hooks, formatting, and diff checks passed.
• Tests use local deterministic servers and existing cassette replay; no live inference engine was run. Linux builds used an existing local static OpenSSL installation.