Config migration that keeps explicit zeros

A service's config is moving from a flat v1 format to a nested v2 format. A coding assistant wrote the migration:

def migrate_config(old):
    return {
        "server": {"host": old.get("host") or "localhost",
                   "port": old.get("port") or 8080},
        "timeout_ms": int((old.get("timeout") or 30) * 1000),
        "retries": old.get("retries") or 3,
        "log_level": "debug" if old.get("debug") else "info",
    }

In staging, the payments worker's config said "retries": 0, because a retried charge can bill a customer twice. After the migration it said "retries": 3. Review the helper against the contract below and write a corrected migrate_config(old).

v1 keys, any of which may be missing: host (string), port (integer), timeout (seconds, a number with at most three decimal places), retries (integer), debug (boolean). A key that is present with the value None (null) counts as missing.

v2 output, always with exactly these keys:

• "server": {"host": ..., "port": ...}
• "timeout_ms": the timeout times 1000, rounded to the nearest whole number
• "retries"
• "log_level": "debug" when debug is true, otherwise "info"
• "dropped": the v1 keys that are not one of the five above, whatever their value, sorted in plain string order (by character code, so uppercase before lowercase), so nothing disappears without a trace

Defaults apply to missing keys only: host "localhost", port 8080, timeout 30, retries 3, debug false. A value that is present is kept even when it is 0, false or an empty string.

Examples:

• migrate_config({"host": "db.internal", "retries": 0, "timeout": 2.5}) returns {"server": {"host": "db.internal", "port": 8080}, "timeout_ms": 2500, "retries": 0, "log_level": "info", "dropped": []}.
• migrate_config({"timeout": 0, "debug": True, "color": "blue", "retries": None}) returns {"server": {"host": "localhost", "port": 8080}, "timeout_ms": 0, "retries": 3, "log_level": "debug", "dropped": ["color"]}.
• migrate_config({}) returns {"server": {"host": "localhost", "port": 8080}, "timeout_ms": 30000, "retries": 3, "log_level": "info", "dropped": []}.