MDL-88218: Emulate common MySQL functions in the SQLite driver

[erseco]

Problem

Running a block_configurable_reports SQL report on the SQLite driver fails with:

Error reading from database
Debug info: SQLSTATE[HY000]: General error: 1 no such function: FROM_UNIXTIME
SELECT id, username, firstname, lastname, FROM_UNIXTIME(timecreated) AS created FROM mdl_user WHERE deleted = 0 ORDER BY id DESC LIMIT 5000

Plugins that accept raw SQL (configurable reports being the canonical example) almost always receive queries written in MySQL dialect, and SQLite has no FROM_UNIXTIME(), DATE_FORMAT(), IF(), etc.

Fix

SQLite supports user defined functions, so configure_dbconnection() now registers emulations of the most common MySQL date/time and string helpers on every connection:

FROM_UNIXTIME, UNIX_TIMESTAMP, DATE_FORMAT, NOW, CURDATE, CURTIME, IF, MD5, CONCAT, CONCAT_WS

Notes:

• Emulations follow MySQL semantics: CONCAT() returns NULL when any argument is NULL (intentionally overriding the SQLite ≥ 3.44 built-in, which skips NULLs), FROM_UNIXTIME(NULL) returns NULL, unknown DATE_FORMAT specifiers yield the literal character.
• DATE_FORMAT/FROM_UNIXTIME(ts, fmt) cover the commonly used format specifiers; week-based specifiers (%u, %v, %V, %x, %X) are approximated with their ISO-8601 equivalents.
• Registration prefers Pdo\Sqlite::createFunction() when available (PHP 8.4+ PDO::connect()) and falls back to PDO::sqliteCreateFunction().
• Moodle core is unaffected: cross-DB SQL goes through sql_concat() & friends; this only rescues raw MySQL-flavoured SQL from plugins.

Testing

Verified against the real driver file with a standalone harness (parent class stubbed, real PDO SQLite connection): the exact failing report query plus 20 function checks, including NULL handling and format-specifier edge cases — all pass. The failing scenario is reproducible in Moodle Playground by loading the configurable_reports blueprint and opening /blocks/configurable_reports/viewreport.php?id=1.

[erseco]

Closing without merging: applied this same commit directly onto MDL-88218-Add-experimental-SQLite-support-for-Moodle-WASM-environments (the branch behind #1) to stay consistent with how the fix was pushed onto the other version branches (#2, #3, #4, #5), instead of leaving it as a separate stacked PR.