feat: download an MCQA sub query's result over the instance tunnel

[TonyStack2026]

Why

SQLExecutor(INTERACTIVE) reads a sub query through the instance tunnel unless the caller opts out, and that is also the path JDBC's MaxQA mode takes. Before this, an interactive consumer fell back to re-running its statement as an offline instance — so #6's own acceptance test had to set useInstanceTunnel(false) to avoid being greened by that fallback. This closes B3, the last item standing between the emulator and a local MaxQA regression run.

Stacked on #6 (this branch is #6 + 1 commit): the diff here shows #6's session commit as well. If you merge #6 first, this PR collapses to a single commit; if you'd rather review them together, review #6 and this one in order.

Contract

Derived from the public Java SDK at the pinned release used by tests/java (0.61.2-public, 1c11317):

GET /projects/{p}/instances/{id}?data&cached&schema_in_stream&taskname=..[&queryid=N]
     [&rowrange=(start,count)][&sizelimit=bytes][&instance_tunnel_limit_enabled]

• No download session exists to ask for a schema, so the stream carries its own: field 1 JSON (TunnelTableSchema shape) → SCHEMA_END_TAG 33553920 → that frame's CRC32C (over the field number as a 4-byte LE int, then the payload — not the tag or the length). The reader resets its checksum there, so the records after the frame are byte-identical to wire.Protobuf — pinned by TestProtobufSchemaTailMatchesPlainStream.
• odps-tunnel-record-count = rows still available from this read's start offset, not the size of the result. SessionRecordSetIterator remembers the first answer's count and pages openRecordReader(offset+cursor, fetchSize) while cursor < count; reporting the total makes an offset read walk off the end. (This is the one number here I could not read off the SDK, only deduce from its loop — the Java case offsetReadPagesTheSubQueryResult is the check.)
• rowrange windows; sizelimit shortens a response by halving (never to zero rows, so a reader always advances); instance_tunnel_limit_enabled applies READ_TABLE_MAX_ROW (10k) to that read only — the sub query keeps its full result.
• Still-running sub query → the read waits (that is what cached asks for), and past 30s answers the SDK's own retryable OdpsTaskTimeout / "Wait for cache data timeout" instead of an empty stream.
• Distinct structured answers, never an empty 200: failed sub query, cancelled (Job is cancelled), unknown queryid, wrong taskname, one-shot instance (InvalidState, points at ?downloads), and a statement with no result set → InstanceTypeNotSupported / "Non select query not supported", the pair SQLExecutorImpl#checkIsSelect looks for. Tunnel errors are JSON now (fail() learned to treat ?cached as a tunnel endpoint) — the XML body left the SDK with no error code to route on.
• Sub queries keep their typed result beside the CSV text, in the same retention window and the same instance byte budget, so both paths of one statement serve the same rows with the same types.

Verification (source 9a372fa, Linux amd64, 2026-09-22 09:3x CST)

Check	Result
go test -race -count=1 ./internal/...	engine / server / wire all ok; 10 new cases in mcqa_download_test.go
go vet -unreachable=false ./..., gofmt -l	clean
McqaSessionTest vs a locally built binary	6/6, incl. defaultFetchDownloadsTheSubQueryResult (typed bigint back from the stream) and offsetReadPagesTheSubQueryResult
Same two cases vs the previous build (#6)	both fail with NoSuchObject: unsupported endpoint — they are not passing via the offline fallback
EmulatorTest 26/26, UdfMetadataTest 4/4	unchanged, no offline-plane regression
tests/python/run.py PyODPS contract probe, fresh process	30 passed / 0 failed
ReliabilityTest	not run locally: testcontainers can't inspect the docker bridge network on this host; it runs in CI

Replay: go test -race ./internal/..., then go build -o /tmp/em ./cmd/emulator && /tmp/em -listen 127.0.0.1:18221 & and mvn -o -f tests/java/pom.xml -Demulator.endpoint=http://127.0.0.1:18221 -Dtest=McqaSessionTest test.

Still not simulated (said so in README / /capabilities)

Compressed or Arrow-encoded sub-query downloads (Protobuf only), named-session attach, MaxQA v2 (/mcqa), per-statement statistics, session-scoped SQL state isolation. subquery-instance-tunnel was added to the mcqa capability list.