fix: make extend_sqlglot idempotent

[sravankumarkunadi]

Fixes #5908.

_override saved whatever was currently installed under _<name>, so calling extend_sqlglot() a second time saved the SQLMesh wrapper as "the original". After that _parse_types called itself and every CAST/type parse in the process raised RecursionError, with sqlglot's real method gone. It doesn't need a deliberate double call: extend_sqlglot() is the first line of sqlmesh/__init__.py, so if anything later in that import fails, the package is evicted while the patched Parser stays behind, and the next import sqlmesh triggers it.

_override now returns early if the wrapper is already installed, and UNWRAPPED_INTERVAL_VALUES gets the same membership guard TRANSFORMS and WITH_SEPARATED_COMMENTS already had (it was growing by two entries per call).

Added test_extend_sqlglot_is_idempotent, which fails on main with the recursion assertion and passes here.

[sravankumarkunadi]

Closing as a duplicate of #5921, which has the same fix and is already approved.