feat(agui): supports multimodality, expandable sessions, request context

[DLancerC]

Description

multimodality

a new content-part model (AguiContentPart / AguiTextContent / AguiImageContent with URL and data-URI sources) plus a backward-compatible AguiMessageDeserializer, letting AguiMessage and AguiMessageConverter carry and forward image inputs to the agent;

expandable sessions

a pluggable AguiSessionManager SPI with InMemoryAguiSessionManager (default, renamed from the previous in-memory impl) and SessionAwareAguiSessionManager (Spring Session based, scalable and restart-safe);

transparent request data

a new AguiRequestContext carrying thread id, headers and raw payload, propagated through AgentResolver / AguiRequestProcessor and the MVC / REST / WebFlux entry points down to the agent. The example app is updated accordingly (WebFluxCodecConfiguration raises the codec limit to 16 MB; index.html gains image upload / URL buttons, previews and inline rendering), and full unit tests are added for the new context, converter, model, adapter and both session managers. To verify: run mvn test on agentscope-extensions-agui and spring-boot-starter-agentscope-agui, then launch the agentscope-examples/agui demo and confirm plain text, image upload/URL, multi-turn session persistence and large-payload (>256KB) requests all work as expected.

Checklist

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

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[codecov]

Codecov Report

❌ Patch coverage is 57.85441% with 110 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ava/io/agentscope/core/agui/model/AguiMessage.java	16.00%	40 Missing and 2 partials ⚠️
...scope/spring/boot/agui/mvc/AguiRestController.java	0.00%	17 Missing ⚠️
.../spring/boot/agui/common/DefaultAgentResolver.java	0.00%	10 Missing ⚠️
...cope/core/agui/converter/AguiMessageConverter.java	73.33%	5 Missing and 3 partials ⚠️
...tscope/spring/boot/agui/mvc/AguiMvcController.java	0.00%	8 Missing ⚠️
...cope/core/agui/processor/AguiRequestProcessor.java	0.00%	6 Missing ⚠️
...e/spring/boot/agui/webflux/AguiWebFluxHandler.java	0.00%	5 Missing ⚠️
...ng/boot/agui/common/AguiAgentAutoRegistration.java	60.00%	3 Missing and 1 partial ⚠️
...scope/core/agui/model/AguiMessageDeserializer.java	88.46%	0 Missing and 3 partials ⚠️
...va/io/agentscope/core/agui/AguiRequestContext.java	92.59%	0 Missing and 2 partials ⚠️
... and 5 more

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