fix(plugin-mysql): read MARIADB_CONST_STRING by length to stop JSON detection flicker

[datlechin]

Summary

User reported audit_log.payload chevron flickering on rapid Cmd+R refresh. Inspector pane confirmed the column type was alternating between json and string across refreshes — same query, same column, intermittent classification.

Root cause

MariaDBPluginConnection.mysqlTypeToString recognizes JSON-stored-as-LONGTEXT by reading MariaDB's extended metadata:

var attr = MARIADB_CONST_STRING()
if mariadb_field_attr(&attr, fieldPtr, MARIADB_FIELD_ATTR_FORMAT_NAME) == 0,
   let str = attr.str, attr.length > 0,
   String(cString: str) == "json" {
    return "JSON"
}

MARIADB_CONST_STRING is a length-prefixed buffer ({ const char *str; size_t length; }) — not null-terminated. String(cString:) scans bytes until it hits the next \0, which means it reads past the four bytes of "json" into whatever memory happens to live next in the result-set buffer.

When that adjacent memory contained a null byte → String(cString:) returned "json" → comparison passed → column tagged JSON → JSON kind → chevron visible.
When that adjacent memory contained non-null bytes → String(cString:) returned "json…garbage" → comparison failed → fell through to the type switch → column tagged LONGTEXT → text kind → no chevron.

The contents of the adjacent memory are non-deterministic (depends on what the connector library wrote there for previous rows / fields, allocator alignment padding, etc.), which is why the bug presented as a flicker rather than a consistent failure.

Fix

Read exactly attr.length bytes:

if mariadb_field_attr(&attr, fieldPtr, MARIADB_FIELD_ATTR_FORMAT_NAME) == 0,
   let str = attr.str, attr.length > 0,
   let value = String(data: Data(bytes: str, count: Int(attr.length)), encoding: .utf8),
   value == "json" {
    return "JSON"
}

Data(bytes:count:) copies exactly count bytes and stops, so there is no over-read. String(bytes:encoding:) (which SwiftLint prefers over the non-failable String(decoding:as:)) returns nil on invalid UTF-8, which we want anyway — anything that isn't valid UTF-8 isn't going to be the literal "json" we're looking for.

Test plan

• Open MySQL tablepro_demo → audit_log.
• Cmd+R 20+ times in rapid succession.
• Inspector pane shows payload: json on every refresh, never string.
• Chevron accessory visible on every payload cell every refresh.
• Click chevron / double-click cell opens JSON editor (Text + Tree tabs).
• Other LONGTEXT columns (not declared as JSON) still classify as LONGTEXT and use the plain text editor.