[FEAT] Cancel server-side query when caller context is cancelled (#126)

[40u5]

What changes were proposed in this pull request?

Fixes #126.

• Adds Interrupt(ctx, interruptType, operationIdOrTag) to the SparkConnectClient interface (spark/client/base/base.go) and implements it on sparkConnectClientImpl (spark/client/client.go), wrapping the proto-level Interrupt RPC and branching on INTERRUPT_TYPE_ALL / TAG / OPERATION_ID.
• ExecutePlanClient now remembers the caller's context.Context, the operation id, and a back-reference to the client. ToTable arms a watcher goroutine that, on callerCtx.Done(), sends InterruptRequest{OPERATION_ID, operationId} using a detached 10 s context so the cancellation actually reaches the server.
• Exposes InterruptAll, InterruptTag, and InterruptOperation on SparkSession (spark/sql/sparksession.go) so users can also kill operations explicitly, mirroring PySpark.
• Updates spark/mocks/mock_executor.go to satisfy the extended interface.

Why are the changes needed?

As reported in #126, dataframe.Collect(ctx) previously dropped the gRPC stream locally when the caller cancelled ctx, but the Spark executor kept running the query until the server's idle timeout (5 min). This addresses all three follow-ups from @grundprinzip's reply on the issue:

1. Surfaces InterruptTag on SparkSession.
2. Surfaces InterruptOperation on SparkSession.
3. Wires up automatic interrupt on client-side context cancellation.

Does this PR introduce any user-facing change?

Yes — additive only:

• SparkSession gains InterruptAll(ctx), InterruptTag(ctx, tag), and InterruptOperation(ctx, operationId), each returning the server-reported list of interrupted operation ids.
• Cancelling the context.Context passed to Collect / ExecutePlan / ExecuteCommand now causes the server-side operation to be interrupted instead of running to idle timeout.

No backward-incompatible breakages for users — the SparkConnectClient interface gains a method, but it has no known external implementers; the in-tree mocks.TestExecutor is updated in the same commit.

How was this patch tested?

Added three unit tests in spark/client/client_test.go:

• TestExecutePlanCancellingContextSendsInterrupt — uses a Recv-blocking stream and verifies that cancelling ctx triggers an InterruptRequest with INTERRUPT_TYPE_OPERATION_ID and the matching operation id within 2 s.
• TestInterruptAllCallsClient — verifies Interrupt(ALL, "") plumbs through.
• TestInterruptOperationCallsClient — verifies Interrupt(OPERATION_ID, opID) plumbs through.

go build ./..., go vet ./..., gofmt -l ./spark (no diff), and go test ./spark/... (all packages PASS) run cleanly. The only failure under go test ./... is the existing internal/tests/integration suite, which requires SPARK_HOME to be set and is unrelated to this change.