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.

src/polygateway/client.py

@@ -13,7 +13,7 @@ import hashlib
 import json
 import random
 import time
-from typing import TYPE_CHECKING, Any, Literal, TypeVar
+from typing import TYPE_CHECKING, Any, Literal
 
 from polygateway.backends.memory.breaker import InMemoryGate
 from polygateway.backends.memory.cache import InMemoryCache
@@ -63,8 +63,6 @@ if TYPE_CHECKING:
         SourceConfig,
     )
 
-_T = TypeVar("_T")
-
 
 def _guard_thinking(
     sources: list[SourceConfig],
@@ -442,7 +440,7 @@ def _build_structured(
         return None, None
 
 
-async def gather_bounded(aws: Iterable[Awaitable[_T]], *, concurrency: int) -> list[_T]:
+async def gather_bounded[T](aws: Iterable[Awaitable[T]], *, concurrency: int) -> list[T]:
     """有界并发 gather(D5 便利函数,替代 VT 手搓 semaphore+gather 样板)。
 
     语义与 `asyncio.gather` 默认一致: 结果保序、首个异常上抛;仅增加并发上限。
@@ -451,7 +449,7 @@ async def gather_bounded(aws: Iterable[Awaitable[_T]], *, concurrency: int) -> l
         raise ValueError("concurrency 必须 ≥ 1")
     sem = asyncio.Semaphore(concurrency)
 
-    async def _run(aw: Awaitable[_T]) -> _T:
+    async def _run(aw: Awaitable[T]) -> T:
         async with sem:
             return await aw