Escape backslashes in MySQL JSON path keys

[ybai08]

The MySQL adapter's escape_json_key/1 escaped keys for the SQL string literal but not for MySQL's JSON path syntax. So after MySQL unescapes the string literal, a backslash in a key reaches the JSON path parser as an escape character. With meta = %{"a\\b" => 1, "a\\nb" => 2, "a\nb" => 3, "a\\" => 4} on MySQL 8.4:

query	before	after
p.meta["a\\b"]	nil	1
p.meta["a\\nb"]	3 (the value for the key that contains a newline)	2
p.meta["a\\"]	error 1064 (invalid JSON path)	4

The fix escapes \ and " for the JSON path first, then applies escape_string/1 for the SQL literal. The output for keys containing " or ' is unchanged, so the existing assertions still pass. I also ran the generated path form directly against MariaDB 11, and it resolves the same keys there.

Checks run:

• mix test: 707 passed (new assertions in the json_extract_path test in myxql_test.exs)
• mix format --check-formatted: clean
• ECTO_ADAPTER=myxql mix test against MySQL 8.4 (docker): the same 11 StorageTest failures with and without this change, because the mysql/mysqldump client binaries aren't installed locally. Nothing else fails.
• Ran the three queries in the table above through TestRepo against MySQL 8.4, before and after the change.

Not run: the earthly CI matrix, and MySQL with NO_BACKSLASH_ESCAPES (the adapter's escape_string/1 already assumes backslash escapes are enabled).