fix(buzz-agent): replay reasoning_content for DeepSeek thinking mode

[olympusbuildz]

Problem

DeepSeek seats on buzz-agent with thinking mode fail on every continuation:

400 Bad Request: The `reasoning_content` in the thinking mode must be passed back to the API.

Reporter saw 79 hard failures across deepseek-v4-flash/pro in two days (#5399). Iterative tool-using roles break; retries burn tokens for nothing.

Root cause

Step	What happens on main
Parse	parse_openai correctly reads message.reasoning_content → LlmResponse.reasoning
UI/ACP	Thought chunks emit that string
History	HistoryItem::Assistant only stored OpenRouter reasoning_details — plain reasoning dropped
Replay	openai_body never wrote reasoning_content back

DeepSeek docs (thinking mode): for requests that carry tools, prior assistant reasoning_content must be fully passed back or the API 400s.

Fix

1. Add reasoning: String on HistoryItem::Assistant (keep reasoning_details).
2. Persist response.reasoning at every assistant history push.
3. In openai_body, emit reasoning_content when non-empty.
4. Charge reasoning.len() in estimated bytes.

Single concern — no image_url gating, no provider feature work.

Why not #4773

block/buzz#4773 targets the same class but is stale vs current HistoryItem (reasoning_details era), bundles supports_image_url, emits only when that message has tool_calls, and has no unit tests. This PR is main-current, #5399-only, tested.

Closest PR search: #4773 (related), #3403 / #4332 (different). none found that closes #5399 on current main with tests.

Test plan

cargo test -p buzz-agent --lib reasoning_content
cargo test -p buzz-agent --lib

• openai_body_omits_reasoning_content_when_empty
• parse_openai_captures_reasoning_content
• openai_body_replays_reasoning_content_on_plain_assistant_when_present
• openai_body_replays_reasoning_content_on_tool_call_assistant
• Full buzz-agent lib suite 484 passed @ head 49fc467 (rebased on current main)
• Full monorepo just ci not run for this change (package lib gate only)

Risk / blast radius

• In scope: buzz-agent history + OpenAI-compat request body only
• Not changed: provider feature flags, image_url gating, non-OpenAI transports

Fixes #5399

Retested after rebase onto current main @ 49fc46796.

Acceptance retest (2026-08-18)

Rebased onto current main. Mini proof @ 7dd78e177: cargo test -p buzz-agent --lib reasoning_content — 4/4.