fix: prevent nil pointer panic when artifact service is not configured

[123456wda]

When the load_artifacts tool is used without configuring an artifact service, ctx.Artifacts() returns a trackedArtifacts wrapping a nil Artifacts interface. Calling List/Load on this wrapper causes a nil pointer dereference panic.

Changes:

• agent/callback_context.go: In NewToolContext and NewCallbackContextWithArtifactTracking, only wrap Artifacts in trackedArtifacts when the underlying service is not nil. This prevents the nil dereference at the source.
• tool/loadartifactstool/load_artifacts_tool.go: Add nil checks for ctx.Artifacts() in appendInitialInstructions and processLoadArtifactsFunctionCall to return a clear error message instead of panicking.
• tool/loadartifactstool/load_artifacts_tool_test.go: Add two test cases covering nil Artifacts scenarios (ProcessRequest with no function call and with a load_artifacts function call).

Fixes #283

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