Pydantic AI 2.53.0 closes a streamed-request limiter leak

A streamed request could retain its concurrency slot after early exit or cross-task release and eventually block all requests sharing the limiter. Version 2.53.0 patches it; v1 is unaffected. Custom limiters must support release from another task, and some nested shared-limiter setups now raise UserError. Test cancellation and reuse when upgrading. ³

Relevant to: Pydantic AI v2 applications using model concurrency limiters and streaming.