fix(agent): retry LLM completion on malformed 2xx JSON body

[tlongwell-block]

Problem

A provider can return HTTP 200 with a truncated JSON body — cleanly closed connection, correct framing, content cut off mid-value. Both LLM HTTP loops treated this as a terminal error on the first attempt: AgentError::Llm("json: EOF while parsing a value"), surfaced as code -32000 at the ACP boundary, killing the agent turn before it produced anything.

Observed live in a tb2.1 bench trial (write-compressor, tb21-twins-1): deepseek via OpenRouter returned a truncated body, the agent died mid-prompt with 0 turns completed, and the trial scored 0 on a provider hiccup.

Meanwhile the same loops already retry timeouts, 429s, 5xxs, 499s, and mid-body stream stalls — a truncated-but-complete body was the one transient upstream fault that fell through to terminal.

Fix

In both post() and openrouter_post() (crates/buzz-agent/src/llm.rs): when the fully-received success body fails serde_json::from_slice, continue the existing retry loop instead of returning terminal — same MAX_RETRIES (3) bound, same backoff_with_jitter. On exhaustion, the error goes through terminal_llm_error so it carries cumulative duration + attempt count like every other retried failure (previously the json: error carried neither).

post_anthropic routes through post(), so Anthropic/OpenAI/Databricks/mesh and OpenRouter are all covered.

Why this cannot re-run a tool call

Hard requirement: tool calls are not idempotent, and this change must not introduce any possibility of replaying one.

1. The retry lives inside the HTTP POST helper, below the parse boundary. Tool calls are only ever extracted from a successfully parsed response value (parse_openai/parse_anthropic/parse_responses, all downstream of these helpers' Ok return). A malformed body never parses, therefore no tool call was ever extracted from it, therefore nothing downstream of it ever dispatched.
2. What is re-sent is the completion request itself — the identical body_bytes captured once at function entry. Sending a completion request executes no tools; it asks the model for the next message.
3. Same safety class as existing behavior. The loop already re-sends this identical request on 429/5xx/timeout/stream-stall; this adds one more transient-fault arm to the same loop with the same bytes.

Tests

Three new tests mirroring the existing 499/dropped-connection fixtures (raw TcpListener stubs):

• post_retries_malformed_json_body_and_succeeds — truncated 200 body on attempt 1, valid JSON on attempt 2; asserts success and exactly 2 server-side requests
• post_exhausts_retries_on_persistent_malformed_json — always-truncated body; asserts exactly MAX_RETRIES attempts and a terminal error carrying json: + cumulative/attempt context
• openrouter_post_retries_malformed_json_body_and_succeeds — same recovery through OpenRouter's separate loop

Full cargo test -p buzz-agent green at e7a5d7b (430 lib + all integration targets, 0 failures); cargo fmt + clippy --all-targets clean.

Originating conversation: buzz-benchmarking channel, thread 397a992d.