### Summary
This PR fixes a Go backend bug where updating agent settings, such as
description, could clear the agent DSL.
Root cause:
PUT /api/v1/agents/:canvas_id only bound the dsl field in Go. When the
frontend submitted settings without dsl, the service still updated the
canvas with an empty DSL value.
Changes:
- Treat agent updates as partial patches.
- Preserve existing DSL when dsl is not present in the request.
- Update only specified user_canvas fields instead of saving the full
row.
- Add a regression test for settings updates preserving DSL.
Test:
`go test ./internal/service ./internal/handler`
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>
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