Add asset state store API to the Java SDK

[FrankYang0529]

• Add client.getAssetStateStore(). byName(name) or byUri(uri) returns an AssetStateStore with get, set, delete and clear. The JVM does not receive the inlets and outlets declared on the @task.stub, so the SDK cannot know which assets the task uses. The task code passes the asset's name or URI instead.
• The supervisor answers ASSET_STORE_NOT_FOUND when the key or the asset does not exist. The get returns null for both cases. For any other error, get throws ApiError.
• byName and byUri reject an empty string.

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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