Fix Nacos 3.2.1 Prompt Registry integration

[logicwu0]

Summary

• create the Nacos AI client from the existing Nacos configuration
• subscribe each configurable prompt by its Prompt Registry key
• apply remote templates and variable defaults on initial load and updates
• keep the legacy aggregate configuration listener for backward compatibility

Tests

• mvn -pl auto-configurations/spring-ai-alibaba-autoconfigure-nacos-prompt -am test

Closes #246

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Render Nacos prompt variables with the right delimiters

When a registry prompt uses Nacos's standard double-brace placeholders like {{name}}, this passes the template unchanged into Spring AI's default PromptTemplate renderer, which expects single-brace placeholders like {name}. In that common Nacos Prompt Registry format, the defaults extracted from PromptVariable do not bind to the remote template, so factory.create(...).render() fails or leaves malformed braces instead of rendering the registry prompt; translate the Nacos placeholders or configure an appropriate renderer before building the PromptTemplate.

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[logicwu0]

Fixed in db1835f. Registry templates now normalize only declared Nacos double-brace placeholders to the single-brace form expected by the Spring prompt renderer. The initial-load and update tests now use the registry format.

[chatgpt-codex-connector]

Handle Nacos prompt deletion events

When a subscribed prompt is deleted from the Nacos Prompt Registry, the prompt-change event carries a null prompt, but this branch returns without changing templates. In environments that withdraw or delete a prompt after it has been loaded, callers keep rendering the last remote version indefinitely instead of falling back to the local template or removing the entry, so the null event should actively clear or restore the cached state.

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[logicwu0]

Fixed in db1835f. The factory now retains the first locally created template as a fallback. A null prompt event restores that fallback, or removes the cached entry when no fallback exists. A deletion regression test covers this behavior.