{184399398}: fix lua stackoverflow on temp table

[emelialei88]

Problem

create_temp_table in lua/sp.c used alloca to allocate the DDL string on the stack, sized by the user-supplied column type string. A malicious or buggy SP passing a huge type (e.g. string.rep('TEXT ', 2000000)) into db:table() would blow the stack and crash the DB.

Changes

• lua/sp.c: validate column name/type from the Lua table — reject NULL or oversized values (name > MAXTABLELEN, type > 64). Replace alloca(len) with malloc(len) so large DDLs go to the heap, with matching free() on all exit paths.
• tests/sp.test/t21.req / t21.req.out: new regression test (ddlbomb) that constructs a 10MB type string and confirms the SP fails cleanly with bad column name or type in 'table' instead of crashing.

Notes

The comment about lua_prepare_sql_with_temp_ddl potentially longjmp-ing and leaking ddl is preserved — the leak is bounded and exceptional, same trade-off as before.

[roborivers]

Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_resume_logicalsc_generated **quarantined**
noresetgen
consumer_non_atomic_default_consumer_generated **quarantined**
reco-ddlk-sql [timeout] **quarantined**

[roborivers]

Cbuild submission: Success ✓.
Regression testing: Success ✓.

The first 10 failing tests are:
sc_timepart **quarantined**
consumer_non_atomic_default_consumer_generated **quarantined**
remotecreate_twopc_generated
remotecreate
reco-ddlk-sql [timeout] **quarantined**