fix: put the tier back where "on" by itself is not on

The minimax wire lost its tier value on the assumption that these models
reason by default, so injecting nothing still reads as "on". T10 measured
the real gateway and the assumption does not hold: MiniMax-M3 with no
reasoning parameter did not reason in 5 of 5 rounds, while all six
strength values worked. Existing downstreams on ENABLE_THINKING=true
went from reasoning to silently not reasoning, and the capability table
cannot catch it because phase 5 lets auto through unconditionally.

Restore on_base to the old {"reasoning_effort": "medium"} verbatim. This
is a stopgap - it hands the tier choice back to the library, which this
work set out to remove. The real fix is to constrain auto by the
capability table, a public behaviour change tracked as issue #21.

The assertions that said "minimax injects no tier on the on-tier" go
back with it; each carries a note on why it moved twice.

src/polygateway/thinking.py

@@ -145,10 +145,12 @@ DEFAULT_CAPABILITIES: Mapping[str, ThinkingCapability] = MappingProxyType(
                 "全部观测到推理,rt 分布完全重叠(minimal 64-124 / low 55-112 / medium 51-104 / "
                 "high 62-128 / xhigh 58-118 / max 57-170)——**它们是'开'的六种写法,不是六个深度档**,"
                 "MiniMax 官方只有开/关两态,配哪一个都一样贵。"
-                "**`auto` 已从清单移除**: minimax 的'开'在 wire 上是 on_base={}(什么参数都不注入),"
-                "而 M3 的默认档实测不推理,故 auto 在这条路上表达不了'开'(N=5 全部未观测到推理)。"
-                "注意 `resolve_thinking` 的 Phase 5 无条件放行 auto,故存量 ENABLE_THINKING=true 仍会"
-                "静默落到不推理——那是 wire 的缺口,能力表堵不住,已记入 T10 报告待单独立项。"
+                "**`auto` 已从清单移除**: 实测当时 minimax 的「开」在 wire 上是 on_base={}"
+                "(什么参数都不注入),而 M3 的默认档实测不推理,故 auto 在这条路上表达不了「开」"
+                "(N=5 全部未观测到推理)。`resolve_thinking` 的 Phase 5 无条件放行 auto,能力表"
+                "堵不住这条,故 2026-09-05 由 wire 侧兜住: on_base 改回 {'reasoning_effort': 'medium'},"
+                "存量 ENABLE_THINKING=true 恢复真开推理(权宜之计,正解见 issue #21)。"
+                "本清单仍不含 auto——它记的是实测结论,不随 wire 的权宜之计变动。"
                 f"历史: {_MEASURED} N=10 同样成立;enable_thinking / thinking:{{type}} 两种写法对本模型"
                 "无效,reasoning_effort 才是真开关(findings 2026-08-25 §3.1/§5)。"
                 "另: 2026-08-25 记录的'MiniMax 这一路已停报 completion_tokens_details'本次**不再成立**"