refactor(server): extract SQL/result interfaces into server/sqlcore so flightclient is duckdb-free

[fuziontech]

Summary

• Step 6 of the binary-split plan, stacked on PR refactor(server): extract Arrow Flight client into server/flightclient #486
• New server/sqlcore/ subpackage with the SQL/result interfaces (RowSet, ExecResult, RawConn, ColumnTyper, QueryExecutor), IsEmptyQuery + StripLeadingComments helpers, and OTELGRPCClientHandler
• server/flightclient is now the first consumer to fully drop its server import — making it duckdb-free at the linkage level
• LocalExecutor / PinnedExecutor / LocalRowSet stay in server/executor.go (standalone-mode-only adapters around *sql.DB, not needed by the CP path)

The win

go list -deps ./server/sqlcore | grep duckdb-go     # empty
go list -deps ./server/flightclient | grep duckdb-go # empty (NEW!)

After this PR, a hypothetical cmd/duckgres-controlplane that imports flightclient plus the other already-extracted leaves (arrowmap, auth, sysinfo, tlscert, ducklake) does not link libduckdb at all. The CP still pulls duckdb-go through its server import for the larger Server / clientConn / Config surface, which PR #7+ tackles.

Backward-compat shims

Type aliases and re-export vars in server/executor.go and a new server/sqlcore_aliases.go keep:

type (
    ColumnTyper   = sqlcore.ColumnTyper
    RowSet        = sqlcore.RowSet
    ExecResult    = sqlcore.ExecResult
    RawConn       = sqlcore.RawConn
    QueryExecutor = sqlcore.QueryExecutor
)

var (
    IsEmptyQuery          = sqlcore.IsEmptyQuery
    OTELGRPCClientHandler = sqlcore.OTELGRPCClientHandler
)

The previously private isEmptyQuery and stripLeadingComments in server/conn.go are now thin one-line wrappers delegating to sqlcore.IsEmptyQuery / sqlcore.StripLeadingComments — so the existing TestIsEmptyQuery / TestStripLeadingComments in conn_test.go pass unchanged.

Test plan

• go build ./... clean
• go build -tags kubernetes ./... clean
• go test -short ./server/sqlcore/... ./server/flightclient/... ./server/... ./controlplane/... — all green
• go list -deps ./server/sqlcore | grep duckdb-go returns empty
• go list -deps ./server/flightclient | grep duckdb-go returns empty (the load-bearing check)

Stack

• ✅ PR refactor(duckdbservice): extract DuckDB-free arrow helpers into arrowmap subpackage #477 (add test user #1): arrowmap extraction
• ✅ PR refactor(server): move DuckLakeConfig + migration into server/ducklake #480 (add test user #1.5): ducklake extraction (parallel)
• ✅ PR refactor: AppendValue split + auth/sysinfo subpackage extraction #482 (Add pg_catalog compatibility and binary format support #2): AppendValue split
• ✅ PR refactor(server): extract auth and sysinfo helpers into pure subpackages #483 (Add connection rate limiting to prevent brute-force attacks #3): auth + sysinfo
• ✅ PR refactor(server): extract TLS / ACME helpers into server/tlscert #484 (Add configurable DuckDB extension loading #4): tlscert
• ✅ PR refactor(server): extract Arrow Flight client into server/flightclient #486 (Update README with new features #5): flightclient — base of this PR
• ⚠️ This PR (Add COPY protocol, graceful shutdown, and enhanced pg_catalog support #6): sqlcore + flightclient is now duckdb-free
• ⏳ PR Fix DuckLake ATTACH command format #7: split server.go / conn.go / types.go's duckdb-bound code into server/exec/ so server itself stops linking duckdb-go (depends on PR add test user #1.5 for the config-type rebase)
• ⏳ PR Add DuckLake with MinIO object storage support #8: introduce cmd/duckgres-controlplane and cmd/duckgres-worker binaries
• ⏳ PR Add release workflow for ARM and x86 builds #9: flip CI/CD + Helm + per-DuckDB-version matrix

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