Pydantic AI 2.23.0 adds real cost tracking to UsageLimits and fixes a Google ADC credential env-var leak

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Released 2026-08-04, v2.23.0 adds `cost` to `RunUsage` and `cost_limit` to `UsageLimits`, so agent runs can be bounded by spend rather than by token counts alone, which matters for anyone budgeting long-horizon loops. It also introduces `ToolAvailabilityDeltaPart` with native `tool_addition` and `additional_tools` rendering for models that change their tool set mid-run. On the security side it fixes `GoogleCloudProvider` credential scoping and an Application Default Credentials environment variable leak, plus a Temporal payload TypeAdapter cache that was bounded and is now unbounded, and async streams that now close on teardown rather than waiting for the garbage collector.