fix: address the independent verification findings (issue #5, #6)

The verifier caught that the disable-direction evidence only proved "no
regression", not "actually took effect": on M3 the disabled runs and the
no-opinion baseline are identically distributed, because that model does
not reason by default anyway. So the disable runs alone cannot rule out
the very failure mode issue #5 is about -- the parameter being silently
dropped upstream. The bogus-value experiment that does rule it out was
sitting in the findings document instead of the test suite; it is now
case L3b, and the L3 assertion that could never fail is gone.

Also from the review: the e2e helper caught bare Exception, which would
have disguised a library bug as an unavailable source, exactly the
silence the reporting discipline exists to prevent; the unregistered
model warning fired on every request instead of once per source; and the
transport caught ValueError broadly enough to mislabel unrelated errors,
now narrowed to a dedicated ThinkingUnsupportedError.

The design and plan still described the original judgement criteria,
which the measurements had already overturned. Both now match what the
tests actually do, and the design no longer claims the only new failure
surface is the openai one -- dissect configures MiniMax-M2.7 with
ENABLE_THINKING=false and will fail at assembly, which has to be
coordinated before this merges.

research-wiki/plans/2026-08-02-thinking-capability.md

@@ -235,9 +235,11 @@ def _fingerprint_mark(s: SourceConfig) -> str:
 
 **三条必须遵守的测试纪律**：
 
-其一，**主判据用 `completion_tokens`，`reasoning_tokens` 只作辅助**。中转在上游不返回 usage 时会本地补算并吃掉 `completion_tokens_details`（findings §4c 实测同一请求 10 轮呈 6:4 双峰），拿它做单轮断言必然 flaky；而 `completion_tokens` 在补算路径下依然有值。
+其一，**判别量只能是 `reasoning_tokens`**。（执行时按 e2e 实测修正：本条初稿写的是「主判据用 `completion_tokens`」，被数据推翻——两档的输出长度分布**重叠**，关闭档实测最高 46、开启档最低 13，按长度阈值判两个方向都会误判。）`completion_tokens` 仅作 `reasoning_tokens` 被中转吃掉时的退路（findings §4c、§2.5）。
 
-其二，**关闭方向要求每轮满足，开启方向只要求多数轮满足**。关掉后 `completion_tokens` 极稳定（实测 4–10），推理量则方差大。
+其二，**关闭方向要求每轮满足，开启方向只要求多数轮满足**。中转吃掉 ctd 时开启方向可能偶尔观测不到，关闭方向不受影响。
+
+其四，**必须有不依赖输出侧噪声的锚点**：L2b 比较两档的 `prompt_tokens`（相对比较，无魔数），L3b 用非法值反证 `none` 是被识别而非被静默丢弃——后者正是 issue #5 的原始故障形态，不排除它，关闭方向的证据就只到「未回归」，够不到「已生效」。
 
 其三，**源不可用必须跳过并在报告中显式记为「未覆盖」**，不得静默计入通过（实测中 kimi 渠道 429 后被中转下线并返回 404）。报告要能一眼看出哪些矩阵行没跑到。