test(coderd/x/chatd): merge the computer-use child CreateChat tests - #30943

The four TestSpawnComputerUseAgent_* tests in subagent_test.go (CreatesChildWithChatMode, SystemPromptFormat, ChildIsListedUnderParent and RootChatIDPropagation) each built a database and a server, created a parent and a computer-use child through Server.CreateChat with the same options, and checked a different part of the result. This PR merges them into one test, TestCreateChat_ComputerUseChildPersistsLineage, that builds the fixture once and keeps every check. The old names suggested spawn_agent coverage, but spawn_agent creates its child through chatstate.CreateChatWithID, and its tests live in subagent_internal_test.go, which this PR does not touch.

Why it is safe

• Every removed assertion is kept. ParentChatID, RootChatID and Mode are checked on both the returned chat and a fresh GetChatByID read, and the test still checks that a system-role message contains the prompt.
• I ran 14 production mutants with go test -overlay against the old tests on main and against the new test. They drop ParentChatID, RootChatID or Mode from the CreateChat input, from the insert or from the returned chat, drop the system prompt or store it with the user role, replace the returned chat with a zero chat, force every message role to user, or blank the message content. The old tests and the new test each catch all 14.

Review focus

The loop over {"returned": child, "fetched": got} runs the same six checks on both sources, and each failure message names its source.

Validation

• go test ./coderd/x/chatd/ -run 'TestSpawnComputerUseAgent|TestCreateChat_ComputerUse' -count=1 -p 4 -parallel 4: passes on main (old names) and on this PR (new name)
• go vet ./coderd/x/chatd/: clean
• golangci-lint run --fix=false ./coderd/x/chatd/: 0 issues
• pre-commit hook: passed

Xum, an AI agent, opened this PR for @ibetitsmike and wrote this description.