fix(knowledge): avoid panic on empty table raw content

[MackDing]

Problem

• uploading or editing some knowledge-base table content could fail with code=105000005 and panic on index out of range [0] with length 0
• root cause: packInsertData accessed slice.RawContent[0] without validating that RawContent existed and contained a valid table payload

Fix

• guard against empty RawContent
• guard against nil/non-table/empty table payload before dereferencing index 0
• return typed business errors instead of panicking so the caller can fail gracefully
• add focused tests covering empty payload, invalid payload, and valid table rows

Test

• PATH=/home/clawos/work/.local-go/go/bin:$PATH GOPATH=/home/clawos/work/.gopath GOCACHE=/home/clawos/work/.gocache GOMODCACHE=/home/clawos/work/.gomodcache GOPROXY=https://proxy.golang.org,direct GOSUMDB=sum.golang.org GOFLAGS='-buildvcs=false -p=1' /home/clawos/work/.local-go/go/bin/go test -gcflags='all=-N -l' ./domain/knowledge/service -run 'Test(EventHandle|PackInsertData)' -count=1 -v

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[MackDing]

Quick follow-up: this PR is ready on my side. If there is a preferred fix direction or any additional test / guard you’d like for the empty-table case, I’m happy to update it quickly. Thanks for reviewing.