### Summary
The Go backend Agent component was not returning artifacts produced by
the CodeExec tool. While the Python agent collects the "`_ARTIFACTS`"
envelope from tool responses and appends artifact markdown to the final
content, the Go agent only returned the assistant text, so generated
images were missing from the chat output.
### Changes
- Wire `react.WithMessageFuture()` in `runEinoReActAgent` and store the
resulting `MessageFuture` in the invocation context.
- After the ReAct loop finishes, drain the future and extract
``_ARTIFACTS`` entries from every tool response message.
- Support reading the tool payload from both `msg.Content` and
`msg.UserInputMultiContent` to match eino's tool contract.
- De-duplicate artifacts by URL and render images as `!` and other files
as download links.
- Add `agent_artifact_test.go` with a regression test that simulates a
CodeExec-style tool response carrying an image artifact and verifies it
is collected and formatted.
### Verification
- `go test ./internal/agent/component/... -run
TestAgent_ReActAgent_CollectsArtifactsFromCodeExecTool` passes.
- `go test ./internal/agent/component/... -count=1` compiles; the only
failure is an unrelated DNS-pinning timeout test
(`TestInvoke_ProxyDNSPin`).
- `gofmt` clean for modified files.
### Related
Fixes the behavior shown in the screenshot where the Go agent ignored
the CodeExec-generated PNG artifact.
8b065d3ddd