feat(cubesql): add disable_post_processing to /v1/cubesql endpoint

[rynmccrmck]

Closes here.

Queries that require DataFusion post-processing fetch intermediate rows from the source DB capped at CUBEJS_DB_QUERY_LIMIT (default 50k). On large datasets this silently truncates the intermediate result, producing wrong aggregates.

The sql4sql (/v1/sql?format=sql) path already classifies plans as pushdown/regular/post-processing and reports it to the caller, but never executes so it cant truncate. The /v1/cubesql path executes and streams results, so to guarantee accuracy it should block before execution.

This adds the same control to /v1/cubesql with a hard guarantee: if the final plan still requires post-processing after the cost model is biased against it, the request fails (the error is delivered in the streamed response body) rather than returning truncated data.

Changes:

• env.ts: register CUBESQL_DISABLE_POST_PROCESSING env var (server-wide default, off by default) so all services are covered without per-request changes
• gateway.ts: read disable_post_processing from request body, fall back to env var, pass through to execSql
• sql-server.ts, index.ts: thread disablePostProcessing param to native
• node_export.rs: set CUBESQL_PENALIZE_POST_PROCESSING_VAR before planning (biases the cost model toward pushdown) and, after planning, fail if the plan root is not a single CubeScan(Wrapped) node

Tests:

• smoke-cubesql.test.ts: error path (asserts the post-processing error is surfaced in the streamed body), push-down success, default-off baseline
• api-gateway/test/index.test.ts: gateway unit test asserting the param is forwarded to execSql; existing /v1/cubesql call-signature assertions updated for the new trailing argument
• backend-shared/test/env.test.ts: unit test for CUBESQL_DISABLE_POST_PROCESSING

Check List

• Tests have been run in packages where changes have been made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required