[OSS-ONLY] Reset pg_strtok state after PLtsql node deserialization

[manisha-deshpande]

Description

This is a follow-up cleanup to the cross-session ANTLR parse tree caching feature added in #4547, which introduced an extension-side node serializer/deserializer (pltsql_stringToNode) that uses the engine's tokenizer via the new pg_strtok_init() setter (added in the corresponding engine PR #733).

Currently, pltsql_stringToNode() calls pg_strtok_init(str) on entry but does not reset the static pg_strtok_ptr before returning. Because the input string lives in a per-statement memory context that gets reset between SQL commands, the tokenizer pointer is left pointing into freed memory until the next caller of stringToNode() overwrites it. With this change, pltsql_stringToNode() resets pg_strtok_ptr to NULL before returning, matching PG's clean-state convention.

Raised during code review of the engine-side pg_strtok_init() addition. Behavior is unchanged in practice - the dangling pointer was never dereferenced because PG's stringToNodeInternal() always overwrites it with its own input before any read (verified via gdb on pg_get_expr / pg_get_constraintdef paths after a cached procedure EXEC). This is a defensive cleanup to remove the stale-pointer footgun for any future caller that might read the static directly.

Issues Resolved

BABEL-6037

Test Scenarios Covered

• Use case based -

• Ran the regression test suite, everything passed as expected, confirming no behavioral change.
• gdb-traced an EXEC of a persistently cached procedure followed by a pg_get_expr call in the same backend; confirmed pg_strtok_ptr is reset to 0x0 after pltsql_stringToNode returns instead of being left dangling, pointing to a wiped memory address.

• Boundary conditions -
  N/A

• Arbitrary inputs -
  N/A

• Negative test cases -
  N/A

• Minor version upgrade tests -
  N/A

• Major version upgrade tests -
  N/A

• Performance tests -
  N/A

• Tooling impact -
  N/A

• Client tests -
  N/A

Check List

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