Ensure proper release of data buffers and handle output stream closure in template processing

[JohnNiang]

Recently, we have detected some memory leak reports. That only happens at connection closed unexpectedly.

This PR ensures proper release of data buffers and handle output stream closure in template processing.

2025-11-05T14:24:45.108+08:00 ERROR 1254622 --- [loomBoundedElastic-33650] io.netty.util.ResourceLeakDetector : LEAK: ByteBuf.release() was not called before it's garbage-collected. See https://netty.io/wiki/reference-counted-objects.html for more information.
Recent access records:
#1:
        io.netty.buffer.AdvancedLeakAwareByteBuf.writeBytes(AdvancedLeakAwareByteBuf.java:617)
        org.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:186)
        org.springframework.core.io.buffer.NettyDataBuffer.write(NettyDataBuffer.java:40)
        org.springframework.core.io.buffer.DataBufferOutputStream.write(DataBufferOutputStream.java:55)
        org.thymeleaf.engine.ThrottledTemplateWriterOutputStreamAdapter.write(ThrottledTemplateWriterOutputStreamAdapter.java:189)
        java.base/java.nio.channels.Channels$WritableByteChannelImpl.write(Channels.java:392)
        java.base/sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:306)
        java.base/sun.nio.cs.StreamEncoder.implWrite(StreamEncoder.java:381)
        java.base/sun.nio.cs.StreamEncoder.implWrite(StreamEncoder.java:357)
        java.base/sun.nio.cs.StreamEncoder.lockedWrite(StreamEncoder.java:158)
        java.base/sun.nio.cs.StreamEncoder.write(StreamEncoder.java:145)
        java.base/sun.nio.cs.StreamEncoder.write(StreamEncoder.java:167)
        java.base/java.io.Writer.write(Writer.java:278)
        org.thymeleaf.engine.ThrottledTemplateWriter.write(ThrottledTemplateWriter.java:185)
        org.thymeleaf.engine.Attribute.write(Attribute.java:192)
        org.thymeleaf.engine.Attributes.write(Attributes.java:464)
        org.thymeleaf.engine.OpenElementTag.write(OpenElementTag.java:150)
        org.thymeleaf.engine.OutputTemplateHandler.handleOpenElement(OutputTemplateHandler.java:142)
        org.thymeleaf.engine.AbstractTemplateHandler.handleOpenElement(AbstractTemplateHandler.java:243)
        run.halo.app.theme.dialect.HaloPostTemplateHandler.handleOpenElement(HaloPostTemplateHandler.java:47)
        org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1576)
        org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
        org.thymeleaf.engine.Model.process(Model.java:300)
        org.thymeleaf.engine.IteratedGatheringModelProcessable.processIterationModel(IteratedGatheringModelProcessable.java:368)
        org.thymeleaf.engine.IteratedGatheringModelProcessable.process(IteratedGatheringModelProcessable.java:294)
        org.thymeleaf.engine.ProcessorTemplateHandler.queueProcessable(ProcessorTemplateHandler.java:2106)
        org.thymeleaf.engine.ProcessorTemplateHandler.handleCloseElement(ProcessorTemplateHandler.java:1642)
        org.thymeleaf.engine.CloseElementTag.beHandled(CloseElementTag.java:139)
        org.thymeleaf.engine.Model.process(Model.java:300)
        org.thymeleaf.engine.StandaloneElementTagModelProcessable.process(StandaloneElementTagModelProcessable.java:114)
        org.thymeleaf.engine.ProcessorTemplateHandler.queueProcessable(ProcessorTemplateHandler.java:2106)
        org.thymeleaf.engine.ProcessorTemplateHandler.handleStandaloneElement(ProcessorTemplateHandler.java:1179)
        org.thymeleaf.engine.StandaloneElementTag.beHandled(StandaloneElementTag.java:228)
        org.thymeleaf.engine.Model.process(Model.java:300)
        org.thymeleaf.engine.OpenElementTagModelProcessable.process(OpenElementTagModelProcessable.java:110)
        org.thymeleaf.engine.ProcessorTemplateHandler.queueProcessable(ProcessorTemplateHandler.java:2106)
        org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1559)
        org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
        org.thymeleaf.engine.TemplateModel.process(TemplateModel.java:155)
        org.thymeleaf.engine.ThrottledTemplateProcessor.process(ThrottledTemplateProcessor.java:235)
        org.thymeleaf.engine.ThrottledTemplateProcessor.process(ThrottledTemplateProcessor.java:200)
        org.thymeleaf.spring6.SpringWebFluxTemplateEngine$StreamThrottledTemplateProcessor.process(SpringWebFluxTemplateEngine.java:724)
        org.thymeleaf.spring6.SpringWebFluxTemplateEngine.lambda$createChunkedStream$2(SpringWebFluxTemplateEngine.java:271)
        reactor.core.publisher.FluxGenerate$GenerateSubscription.slowPath(FluxGenerate.java:271)
        reactor.core.publisher.FluxGenerate$GenerateSubscription.request(FluxGenerate.java:213)
        reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.request(FluxContextWrite.java:136)
        reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)
        reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
        reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
        reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)
        reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onSubscribe(FluxContextWrite.java:101)
        reactor.core.publisher.FluxGenerate.subscribe(FluxGenerate.java:85)
        reactor.core.publisher.InternalFluxOperator.subscribe(InternalFluxOperator.java:68)
        reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
        reactor.core.scheduler.BoundedElasticThreadPerTaskScheduler$SchedulerTask.run(BoundedElasticThreadPerTaskScheduler.java:1013)
        java.base/java.lang.VirtualThread.run(VirtualThread.java:329)

Fix the potential memory leak when processing template

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[JohnNiang]

I'm going to merge this PR manually due to no more feedback.