chore: require python 3.12 and adopt PEP 695 type parameters

The telemetry write budget needs asyncio.timeout, whose uncancel accounting
was only fixed after 3.11.1 — pinning the floor at 3.12 removes that hazard
instead of working around it.

Raising ruff's target-version turns on UP047, so gather_bounded,
_anext_within and stream_with_liveness_timeouts move to def f[T](...) and
the two module-level TypeVars go away. That syntax is a SyntaxError on
3.11, so it can only land together with the version bump.

pyproject.toml

@@ -9,7 +9,7 @@ description = "PolyGateway：实验室统一的大语言模型（LLM/VLM/OCR）
 # registry 包页面的正文只认这一项:缺了页面就是一片空白(1.1.2 的教训,twine 会警告
 # long_description missing 但不阻塞上传)。README 在打包时被固化进产物,发布后再改无效。
 readme = "README.md"
-requires-python = ">=3.11"
+requires-python = ">=3.12"
 dependencies = [
     "httpx>=0.27",
     "pydantic>=2.8",
@@ -56,7 +56,7 @@ markers = [
 ]
 
 [tool.ruff]
-target-version = "py311"
+target-version = "py312"
 line-length = 100
 
 [tool.ruff.lint]