Fix targeted Iceberg metadata loading

[EDsCODE]

Summary

• Target Lakekeeper column metadata preloads to simple information_schema.columns table_schema/table_name predicates.
• Skip stale Lakekeeper 404 table metadata only during broad metadata refreshes, while preserving hard failures for requested missing tables.
• Add regression tests and document the Iceberg information_schema behavior in the Lakekeeper runbook.

Test plan

• go test ./server/icebergmeta ./server -run 'Iceberg|InformationSchemaColumnsCompat|Metadata' -count=1
• git diff --check

Notes

• just lint could not run locally because golangci-lint is not installed in this environment.