sql: distSQLSpecExecFactory.ConstructVectorSearch not using InitAllowingExternalRowData

In [this slack conversation](https://cockroachlabs.slack.com/archives/CV581CE78/p1746469163185869?thread_ts=1746286049.818619&cid=CV581CE78), @andy-kimball pointed out that `sql.(*distSQLSpecExecFactory).ConstructVectorSearch` is using `span.(*Builder).Init` instead of `span.(*Builder).InitAllowingExternalRowData` and so will get an internal error when used on external row data.

This is pretty tough to hit. It requires `SET experimental_distsql_planning = always;` + vector search + external row data. Here's a repro using `cockroach demo`:

```sql
-- 1. create a table with a vector index

SET CLUSTER SETTING feature.vector_index.enabled = on;

CREATE TABLE abc (
a INT PRIMARY KEY,
b INT,
c VECTOR(3),
VECTOR INDEX (b, c)
);

INSERT INTO abc VALUES (1, 2, '[1, 2, 3]');
ANALYZE abc;
SELECT * FROM abc WHERE b = 2 ORDER BY c <-> '[1, 2, 3]' LIMIT 1;

-- 2. connect to the system tenant and start replication to a new tenant

\demo ls
\connect ''

CREATE VIRTUAL CLUSTER standby FROM REPLICATION OF demoapp ON 'demo://system' WITH READ VIRTUAL CLUSTER;
-- wait until the standby-readonly tenant data_state is ready
SHOW VIRTUAL CLUSTERS;

-- 3. connect to the standby-readonly tenant and try querying the table

\connect ''

SHOW TABLES;
SET experimental_distsql_planning = always;
SELECT * FROM abc WHERE b = 2 ORDER BY c <-> '[1, 2, 3]' LIMIT 1;
```

The internal error looks like this:

```
demo@[local:/Users/michae2/.cockroach-demo]:26257/standby-readonly/defaultdb> SELECT * FROM abc WHERE b = 2 ORDER BY c <-> '[1, 2, 3]' LIMIT 1;
ERROR: internal error: abc uses external row data
SQLSTATE: XX000
DETAIL: stack trace:
pkg/sql/span/span_builder.go:49: Init()
pkg/sql/distsql_spec_exec_factory.go:1471: ConstructVectorSearch()
bazel-out/darwin_arm64-fastbuild/bin/pkg/sql/opt/exec/explain/plan_gist_factory.og.go:1223: ConstructVectorSearch()
bazel-out/darwin_arm64-fastbuild/bin/pkg/sql/opt/exec/explain/explain_factory.og.go:2108: ConstructVectorSearch()
pkg/sql/opt/exec/execbuilder/relational.go:3979: buildVectorSearch()
pkg/sql/opt/exec/execbuilder/relational.go:273: buildRelational()
pkg/sql/opt/exec/execbuilder/relational.go:2733: buildLookupJoin()
pkg/sql/opt/exec/execbuilder/relational.go:228: buildRelational()
pkg/sql/opt/exec/execbuilder/relational.go:1152: buildProject()
pkg/sql/opt/exec/execbuilder/relational.go:203: buildRelational()
pkg/sql/opt/exec/execbuilder/relational.go:2153: buildTopK()
pkg/sql/opt/exec/execbuilder/relational.go:213: buildRelational()
pkg/sql/opt/exec/execbuilder/builder.go:380: build()
pkg/sql/opt/exec/execbuilder/builder.go:298: Build()
pkg/sql/plan_opt.go:936: runExecBuilder()
pkg/sql/plan_opt.go:285: runExecBuild()
pkg/sql/plan_opt.go:268: makeOptimizerPlan()
pkg/sql/conn_executor_exec.go:3308: makeExecPlan()
pkg/sql/conn_executor_exec.go:2855: dispatchToExecutionEngine()
pkg/sql/conn_executor_exec.go:1080: execStmtInOpenState()
pkg/sql/conn_executor_exec.go:172: func2()
pkg/sql/conn_executor_exec.go:4477: execWithProfiling()
pkg/sql/conn_executor_exec.go:171: execStmt()
pkg/sql/conn_executor.go:2342: func1()
pkg/sql/conn_executor.go:2347: execCmd()
pkg/sql/conn_executor.go:2264: run()
pkg/sql/conn_executor.go:1048: ServeConn()
pkg/sql/pgwire/conn.go:252: processCommands()
pkg/sql/pgwire/server.go:1197: func4()
src/runtime/asm_arm64.s:1223: goexit()

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```

Jira issue: CRDB-50399