refactor(appkit): split SQLWarehouseConnector into submit/get/poll/transform

`SQLClient.executeStatement` was a single block: submit the SQL, poll
until terminal, transform the Arrow payload to JSON. Splitting it into
four narrower public APIs lets durable executors compose them without
holding the orchestrator open across the wait:

- `submitStatement(sql, params, opts)` — POST `/sql/statements`,
  returns the raw initial response. Adds a dedicated `sql.submit` span.
- `getStatement(id)` — GET `/sql/statements/{id}`, single status read.
- `pollStatement(id, opts)` — block until the statement reaches a
  terminal state (SUCCEEDED / FAILED / CANCELED / CLOSED), respecting
  the same timeout, signal, and error semantics the old monolithic
  method had.
- `transformResult(response)` — Arrow → JSON row transform, no I/O.

`executeStatement(...)` is preserved and now composes the four publics
(`submit` → `poll` → `transform`). No private wrapper-only helpers
remain. Every error path, abort branch, and status state machine of
the old method is exercised by the new per-API test suites (21 new
tests against `submit` / `get` / `poll` / `transform`).

Motivation (documented inline in JSDoc): durable callers — e.g. a
future TaskFlow-based analytics handler — emit a `statement_submitted`
event with the warehouse-side statement ID right after `submitStatement`
returns, so on crash recovery they can re-attach via `pollStatement`
without re-running the SQL. The TaskFlow integration itself is not in
this PR.

`executeStatement`'s contract is unchanged; analytics (the only external
caller) keeps working without modification. The added `sql.submit` span
is purely additive for OTLP collectors.

Verified: pnpm -r typecheck, pnpm build, full pnpm test
(122 files, 2276 tests) all green.

Signed-off-by: ditadi <victordperd@gmail.com>

Author: ditadi