Address WordPress PHPUnit test fails: charset detection, length validation etc

[chloe-pomegranate]

Summary

Fixes (some of) #320 - WordPress PHPUnit test failures related to charset/collation handling, column length validation, and db capabilities

Changes Made

New Methods

• get_col_charset($table, $column) - Returns charset for a table column by parsing the Collation field from SHOW FULL COLUMNS output
• get_col_length($table, $column) - Returns column length information for validation, supporting char/varchar/binary/varbinary and TEXT/BLOB
• has_cap($db_cap) - Reports supported capabilities: collation, group_concat, subqueries, set_charset, utf8mb4, utf8mb4_520, identifier_placeholders
• determine_charset($charset, $collate) - Upgrades utf8 to utf8mb4 and handles collation mapping

Existing Methods Changed

• query() - Added strip_invalid_text_from_query() validation to reject queries with invalid data for the column charset
• close() - Properly sets ready=false and has_connected=false when closing the connection

Tests

Fixes from the issue:

• Charset detection tests
• Length validation tests
• Capability tests
• Charset tests
• Data handling tests

Other

Haven't touched these ones as they're generally all MySQL specific or SQLite handles them differently:

• test_replace - MySQL REPLACE returns affected rows differently than SQLite
• test_close - Similar to above
• test_mysqli_flush_sync - MySQL-specific, don't think we can do anything here
• test_spatial_indices - SQLite doesn't support SPATIAL KEY
• test_wp_update_comment
• test_strip_invalid_text
• test_set_charset_changes_the_connection_collation

Notes

• For new methods I've used @since 2.3.0 - can change to whatever :)