MDL-88218: Add experimental SQLite support for Moodle WASM environments (4.5)

[erseco]

Summary

Companion to #1 (main), #2 (5.1) and #3 (5.0). Moodle 4.5 still ships the
legacy SQLite PDO driver (lib/dml/sqlite3_pdo_moodle_database.php +
lib/ddl/sqlite_sql_generator.php) that was removed on 5.0+ — which is why the
newer branches add a modernised driver, while 4.5 only needs the existing one
repaired.

That legacy driver was left behind as the DML/DDL base classes evolved, so on
current 4.5 it can no longer install a site (admin/cli/install_database.php
fails). This PR makes it work again (experimental; WASM / demo / testing only).

Fixes

#	File	Problem	Fix
1	lib/ddl/sqlite_sql_generator.php	4.5's sql_generator declares getCreateTempTableSQL() abstract; the generator omits it, so the class is abstract and cannot be instantiated (silent exit 255).	Implement getCreateTempTableSQL() (CREATE TEMPORARY TABLE, mirroring mysql_sql_generator).
2	lib/ddl/sqlite_sql_generator.php	Constructor __construct($mdb) drops the $temptables argument that get_manager() passes → $this->temptables is null → Call to a member function is_temptable() on null.	Forward $temptables to the parent constructor.
3	lib/dml/sqlite3_pdo_moodle_database.php	4.5 expects each driver to create its temptables controller in connect(); this driver never does → $this->temptables null.	Override connect() to initialise moodle_temptables.
4	lib/dml/sqlite3_pdo_moodle_database.php	fetch_columns(): array returns false when a table's CREATE TABLE isn't found → TypeError on PHP 8.	Return an empty array.
5	lib/dml/pdo_moodle_database.php	get_records_sql() / get_records_select() return false on an empty result, but the documented contract (and callers such as license install, which does array_map()) expect an empty array.	Return array() when there are no rows.
6	admin/environment.xml	The <MOODLE version="4.5"> block does not list the sqlite database vendor, so install_database.php's environment check rejects it.	Declare <VENDOR name="sqlite" version="3.0" /> in the 4.5 block.

Verification

Built Moodle 4.5.12 with these changes on top of an erseco/alpine-moodle
image and ran a full CLI install on SQLite:

• admin/cli/install_database.php completes (496 tables, admin + guest users created).
• The site serves over HTTP and a Moodle-context moosh role-list works.
• php -l clean on all changed files; environment.xml is well-formed.

Notes

• No new files: 4.5 already carries the driver, so this is a focused repair
  (4 files, +40/−5).
• pdo_moodle_database.php is the shared PDO base, but SQLite is its only
  consumer here; the empty-array change also matches the method's own docblock.