feat: collect reasoning_tokens from the provider usage payload (issue #6)

Reasoning tokens are already counted inside completion_tokens, so the
cost total was never wrong -- what was missing is the attribution: how
much of a call was spent thinking rather than answering.

LLMResponse and TransportResult each gain a trailing reasoning_tokens
field, and the telemetry port grows from 21 to 22 columns with the new
column appended in both backends so fresh and migrated schemas keep the
same physical order.

None means this particular call did not report the field, not that the
source never reports it: a relay that falls back to a local tokenizer
replaces the whole usage object and drops completion_tokens_details.
Downstream checks must therefore read "in (None, 0)"; no provider was
observed reporting a literal zero.

src/polygateway/middleware/retry.py

@@ -438,6 +438,7 @@ class RetryMW:
             usage_source=result.usage_source,
             cached_prompt_tokens=result.cached_prompt_tokens,
             model_reported=result.model_reported,
+            reasoning_tokens=result.reasoning_tokens,
         )
 
     async def _settle_and_release(self, permit: Permit, actual: int) -> None:

src/polygateway/middleware/telemetry.py

@@ -64,6 +64,7 @@ class TelemetryEmitter:
             error=error,
             cached_prompt_tokens=response.cached_prompt_tokens if response else None,
             model_reported=response.model_reported if response else None,
+            reasoning_tokens=response.reasoning_tokens if response else None,
             # 唯一有"生效源"的入口,故是唯一能并上 extra_body 的(设计决策 D)
             sampling=canonical_sampling_json(merge_sampling(source.extra_body, request.sampling)),
         )
@@ -90,6 +91,7 @@ class TelemetryEmitter:
             # 统计供应商缓存命中率必须带 WHERE cache_hit = false,否则重复计数。
             cached_prompt_tokens=response.cached_prompt_tokens,
             model_reported=response.model_reported,
+            reasoning_tokens=response.reasoning_tokens,
             # 由最外层 TelemetryMW 调用,手上没有 source。缓存命中行无损:
             # sampling 已进缓存 key,能命中即意味调用级参数与历史那次逐字相同
             sampling=canonical_sampling_json(request.sampling),
@@ -117,6 +119,7 @@ class TelemetryEmitter:
             error=error,
             cached_prompt_tokens=None,
             model_reported=None,
+            reasoning_tokens=None,
             # 无具体源,与 model/provider/source_name 置空同一先例(设计决策 D)
             sampling=canonical_sampling_json(request.sampling),
         )
@@ -142,6 +145,7 @@ class TelemetryEmitter:
         cached_prompt_tokens: int | None,
         model_reported: str | None,
         sampling: str | None,
+        reasoning_tokens: int | None,
     ) -> None:
         try:
             # 成本换算(M2 §6): 成功行按单价换算;缓存命中 0.0(未产生新调用);
@@ -182,6 +186,7 @@ class TelemetryEmitter:
                 cached_prompt_tokens=cached_prompt_tokens,
                 model_reported=model_reported,
                 sampling=sampling,
+                reasoning_tokens=reasoning_tokens,
             )
         except asyncio.CancelledError:
             raise