feat(kernel): send per-statement query tags on the kernel backend

[rahuls-db]

What

Brings the kernel (SEA) backend to parity with the Thrift backend for
per-statement query tags. The Thrift backend already reads tags from the
request context and sends them via confOverlay["query_tags"]; the kernel backend
ignored them because the kernel C ABI had no per-statement tags setter. It now does
(kernel_statement_set_query_tags, merged as
databricks-sql-kernel 21504eae), so this wires it up.

No new user-facing API — the same driverctx.NewContextWithQueryTags(ctx, …)
that already works on Thrift now also works on the kernel backend.

How

• internal/backend/kernel/operation.go: after binding parameters and before
  execute, read driverctx.QueryTagsFromContext(ctx), serialize with the shared
  querytags.Serialize, and pass the wire string to kernel_statement_set_query_tags.
  Empty/absent → skipped.
• Interior-NUL guard (internal/backend/kernel/querytags_nul.go):
  checkQueryTags/errQueryTagsNUL rejects a serialized tags string with an
  interior NUL before the length-less C setter would silently truncate it — the
  query-tags counterpart to checkQueryText (set_sql) and checkParamValue
  (bind_parameter), keeping the kernel path at parity with the whole-string Thrift
  path. Pure Go, unit-tested under CGO_ENABLED=0 (querytags_nul_test.go).
• KERNEL_REV + committed header: bumped to 21504eae and the C-ABI header
  synced to that rev. The databricks_kernel CI leg source-builds the kernel .a
  from KERNEL_REV (make kernel-lib), so this is what lets it link the new setter.
  The header sync also carries the other C-ABI decls that landed since the prior pin
  (max_connections, telemetry circuit breaker, a result-stream canceller type, the
  kernel_session_test timeout param); the Go backend calls none of them, so no
  binding code changes were needed.

Note on the published bindings

go.mod still pins the databricks-sql-kernel-bindings modules at v1.0.0 — a
matching bindings release (built from a kernel rev ≥ 21504eae) is a fast-follow
for go get -tags databricks_kernel consumers. CI and this repo's tagged tests build
the kernel from KERNEL_REV and are unaffected; the pure-Go default build never
touches the kernel.

Validation (local)

Built the kernel static/shared lib from 21504eae (the new KERNEL_REV) and, with
the synced header, ran the databricks_kernel-tagged path against it:

• go build / go test (-tags databricks_kernel, CGO on) — pass.
• checkQueryTags guard test under CGO_ENABLED=0 — pass.
• Default pure-Go go build ./... + gofmt — clean.
• Live warehouse E2E: connected on the kernel backend with statement-level tags
  in context and ran a tagged query end to end — statement id
  01f1a7e7-5834-15c7-bd90-bb1c7e0f144d on warehouse 153fb3a25b8051a7.

This pull request and its description were written by Isaac.

[peco-review-bot]

Verdict: 1 Medium

Solid, well-scoped wiring that mirrors the Thrift query-tags path closely (same source, serializer, empty-string skip) and handles kernel error/cleanup correctly. One medium concern: unlike SQL text and bound params, the serialized query-tags string reaches a length-less kernel C ABI setter without the interior-NUL guard this repo explicitly maintains for parity — a NUL would silently truncate tags on the kernel path while Thrift sends them whole.

[peco-review-bot]

✅ No issues identified by the review bot.

[peco-review-bot]

Verdict: 1 Low

Looks good — a faithful port of the Thrift per-statement query-tags behavior to the kernel backend, reusing the shared serializer and mirroring the existing set_sql/bind_parameter NUL guards; C-string lifetimes and error/eviction handling are correct. One low-severity heads-up: the tagged build now needs a symbol only present at KERNEL_REV≥21504eae while go.mod still pins bindings at v1.0.0 (acknowledged fast-follow in the PR).

Other findings

• 🔵 Low — The tagged build now references kernel_statement_set_query_tags, a symbol added at KERNEL_REV=21504eae, but go.mod still pins the published databricks-sql-kernel-bindings modules at v1.0.0 (built from a pre-21504eae rev). Any consumer running go get -tags databricks_kernel against the prebuilt v1.0.0 archives will hit an undefined-symbol link failure — the setter is not in those .a files. CI and this repo's tagged tests source-build from KERNEL_REV (make kernel-lib) and are unaffected, and the default pure-Go build never touches the kernel, so the blast radius is limited to opt-in prebuilt-bindings consumers.

The PR description explicitly acknowledges this as an intended fast-follow, so this is a heads-up rather than a blocker: the matching bindings release must land before this new capability is usable via prebuilt archives, otherwise the tagged build breaks for those users.