feat(agent,acp): wire provider total_tokens through NIP-AM publish chain

[wpfleger96]

What

Wires genuine provider-reported total_tokens through the full buzz-agent → buzz-acp publish chain so kind-44200 events carry real per-turn and cumulative totals for OpenAI-backed models, while preserving all existing behaviour for Anthropic and external harnesses (goose, claude-code).

Why

Live prod data showed 0 of 1,934 archived reports carry totalTokens. Both hardcoded total_tokens: None in pool.rs and the absent field in buzz-agent's parser are root causes. This is the backend half of a two-track fix; the display-fallback half lands in #2035.

Changes

crates/buzz-agent/src/types.rs

• Added total_tokens: Option<u64> to LlmResponse with an explicit doc comment that NIP-AM forbids deriving it.
• Added TurnTotalState enum (Unseen | Exact(u64) | Unknown) with fold() and exact_value() — the tri-state accumulator that distinguishes not-yet-observed from permanently poisoned.

crates/buzz-agent/src/llm.rs

• parse_responses and parse_openai: read usage.total_tokens from OpenAI Chat Completions (including Databricks routes) and the Responses API via sum_usage.
• Anthropic: explicit total_tokens: None — no genuine total available; NIP-AM forbids summing categories.

crates/buzz-agent/src/agent.rs

• Added turn_total_state: &'a mut TurnTotalState to RunCtx.
• Fold response.total_tokens into the accumulator after each usage-bearing response; non-usage-bearing responses (keepalive/stream frames) do not poison.

crates/buzz-agent/src/lib.rs

• Added accumulated_total_state: TurnTotalState to Session (default Unseen).
• Per-turn state passed to RunCtx, folded into session cumulative after each turn.
• Emits accumulatedTotalTokens in usage_update only when cumulative is Exact(n).

crates/buzz-acp/src/usage.rs

• Added accumulated_total_tokens: Option<u64> (serde default) to UsageUpdatePayload — optional for goose compat.
• Added last_total: Option<u64> to SessionState.
• Added turn_total_tokens and cumulative_total_tokens to TurnUsage (field-local — never affect delta_reliable).
• Derive turn-total delta only when prev and current are both Some and monotonic; absence, decrease, or no baseline leaves only the total delta null without touching input/output reliability.

crates/buzz-acp/src/pool.rs

• Replaced both hardcoded total_tokens: None in publish_agent_turn_metric with usage.turn_total_tokens and usage.cumulative_total_tokens.

Tests

20 new tests across the four touched files:

File	Tests
types.rs	TurnTotalState fold, accumulation, exact_value, default (7 tests)
llm.rs	Chat present/absent, Responses present/absent, Anthropic always-None (5 tests)
usage.rs	First turn no baseline, second-turn delta, cumulative decrease (field-local), current absent, goose-shaped deserialization, baseline absent (6 tests)
pool.rs	Exact turn+cumulative mapping, null totals never derived (2 tests)

cargo test -p buzz-acp -p buzz-agent — all passing, 0 failures.

Scope

Boundary: crates/buzz-agent/** + crates/buzz-acp/** only. Desktop unchanged.
costUsd explicitly out of scope.

Related: #2035