fix: preflight schema check prevents duplicate column errors on upgrade

[bcordis]

Summary

Fixes installation failure when upgrading between versions caused by MySQL ALTER TABLE ADD COLUMN failing on columns that already exist from a previous partial upgrade attempt.

Problem

MySQL's ALTER TABLE ADD COLUMN is not idempotent — it throws "Duplicate column" if the column already exists. Joomla's schema updater aborts on the first SQL error and marks the install as failed, but the component files have already been copied. This leaves the site in a broken state: new PHP code but old database schema.

This happens when:

• A previous upgrade attempt failed partway through (columns partially added)
• A database was imported/restored from a different version
• Joomla retries the schema update after a partial failure

Fix

New ensureSchemaReady() method runs in preflight() before Joomla copies files or runs the schema updater:

1. Reads the currently installed install.mysql.utf8.sql (still on disk from the old version)
2. Parses CREATE TABLE statements to extract expected columns per table
3. Compares against the live database
4. Drops any columns that exist in the DB but are NOT in the install SQL — these are leftovers from a failed partial upgrade
5. DB is now clean for the installed version → Joomla's migration SQL runs without errors

Why this approach

• No hardcoded column lists — the install SQL file IS the schema reference for the installed version
• Self-maintaining — each version's install SQL automatically defines what's expected
• Works for any version combination — 10.0.1→10.2.0, 10.1.0→10.3.0, retry after failure, etc.
• Also fixes DB import/restore scenarios where the schema doesn't match expectations
• Preserves dbfix tool — <schemas> stays in XML, migration SQL files unchanged

Flow

preflight() → parse OLD install.sql → diff vs live DB → drop extras
    ↓
Joomla copies new files (including new install.sql)
    ↓
Joomla schema updater runs migration SQL → ADD COLUMN succeeds
    ↓
postflight() → scripture migration, guided tours, etc.

Test plan

• Fresh install on empty database — no errors
• Upgrade 10.0.1 → 10.2.x — no duplicate column errors
• Simulate partial upgrade failure (add some columns manually) → retry upgrade → completes cleanly
• Import 10.2.x database into 10.0.x install → upgrade → no errors
• Joomla dbfix tool still shows correct schema status
• All 538 PHPUnit tests pass
• PHP syntax clean

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