fix(backend/kernel): defer databricks-sql-kernel poetry dep declaration

CI is failing across all jobs at \`poetry lock\` time:

    Because databricks-sql-connector depends on databricks-sql-kernel
    (^0.1.0) which doesn't match any versions, version solving failed.

The kernel wheel isn't yet published to PyPI — we verified the name
is available via the Databricks proxy, but the package itself hasn't
been built and uploaded yet. Declaring it as a poetry dep (even an
optional one inside an extra) requires the version to be resolvable,
and \`poetry lock\` runs as the setup step for every CI job: unit
tests, linting, type checks, all of them.

Fix: drop the \`databricks-sql-kernel\` dep declaration and the
\`[kernel]\` extra from pyproject.toml until the wheel is on PyPI.
The lazy import in \`backend/kernel/client.py\` still raises a
clear ImportError pointing at \`pip install databricks-sql-kernel\`
(or local maturin) when use_sea=True is invoked without the kernel
present.

When the kernel is published, a small follow-up will add back:

    databricks-sql-kernel = {version = "^0.1.0", optional = true}
    [tool.poetry.extras]
    kernel = ["databricks-sql-kernel"]

A pointed comment in pyproject.toml documents the deferred change.

Co-authored-by: Isaac
Signed-off-by: Vikrant Puppala <vikrant.puppala@databricks.com>

Author: vikrantpuppala

pyproject.toml

@@ -32,16 +32,24 @@ pyarrow = [
 pyjwt = "^2.0.0"
 pybreaker = "^1.0.0"
 requests-kerberos = {version = "^0.15.0", optional = true}
-# Optional kernel backend: `pip install 'databricks-sql-connector[kernel]'`
-# unlocks use_sea=True, which routes through the Rust kernel via PyO3.
-# Without it, use_sea=True raises a pointed ImportError. The kernel
-# wheel itself ships from the databricks-sql-kernel repo.
-databricks-sql-kernel = {version = "^0.1.0", optional = true}
 
 
 [tool.poetry.extras]
 pyarrow = ["pyarrow"]
-kernel = ["databricks-sql-kernel"]
+# `[kernel]` extra is intentionally not declared here yet.
+# `databricks-sql-kernel` is built from the databricks-sql-kernel
+# repo and not yet published to PyPI; declaring it as a poetry dep
+# breaks `poetry lock` for every CI job. Once the wheel is on PyPI
+# the extra will be added back here:
+#
+#     databricks-sql-kernel = {version = "^0.1.0", optional = true}
+#     [tool.poetry.extras]
+#     kernel = ["databricks-sql-kernel"]
+#
+# Until then, install the kernel separately:
+#     pip install databricks-sql-kernel
+# or (local dev):
+#     cd databricks-sql-kernel/pyo3 && maturin develop --release
 
 [tool.poetry.group.dev.dependencies]
 pytest = "^7.1.2"

src/databricks/sql/backend/kernel/client.py

@@ -66,9 +66,13 @@
 try:
     import databricks_sql_kernel as _kernel  # type: ignore[import-not-found]
 except ImportError as exc:  # pragma: no cover - import-time error surfaces clearly
+    # The `databricks-sql-kernel` wheel is not yet on PyPI, so we
+    # don't yet declare it as an optional extra in pyproject.toml
+    # (doing so breaks `poetry lock`). Once published the install
+    # hint will move to `pip install 'databricks-sql-connector[kernel]'`.
     raise ImportError(
         "use_sea=True requires the databricks-sql-kernel package. Install it with:\n"
-        "  pip install 'databricks-sql-connector[kernel]'\n"
+        "  pip install databricks-sql-kernel\n"
         "or for local development from the kernel repo:\n"
         "  cd databricks-sql-kernel/pyo3 && maturin develop --release"
     ) from exc