adapter: track ALTER SOURCE's source in the purified statement's dependencies

[tonydu-mz]

Motivation

ALTER SOURCE is purified off the coordinator thread. Its target source is
carried in the AST as an UnresolvedItemName, so name resolution never records
it and the statement's resolved_ids, from which PlanValidity's dependency
set is built, does not contain it.

A source dropped concurrently with that off-thread purification therefore
passed the validity check when the purified statement came back, and planning
then called get_entry on the missing id, panicking the coordinator with
"catalog out of sync" (plan_purified_alter_source_add_subsource,
plan_purified_alter_source_refresh_references).

Description

Extend the purified statement's dependency ids with the source that
mz_sql::pure::statement_source resolves (introduced in #38564, where the same
resolution feeds the pre-purification authorization check). With the id
tracked, PlanValidity::check detects the drop and the coordinator repurifies
the original statement, ending in a clean unknown-item error instead of a
panic. For CREATE TABLE ... FROM SOURCE the source is already in
resolved_ids, so the extension is a no-op there.

Verification

The race needs a drop landing between purification spawn and completion; there
is no deterministic hook for that in sqllogictest. Verified by code trace:
PlanValidity::check uses try_get_entry and returns
ConcurrentDependencyDrop for a missing id, and the PurifiedStatementReady
handler repurifies on validity failure. ALTER SOURCE IF EXISTS on a
never-resolving name contributes no phantom dependency (statement_source
returns None).

Stacked on #38564.