[Bug] Quoting in the string-to-MAP cast rule cannot express an empty or literal-null key or value

The three string-to-collection cast rules share a mini-language where quotes group a token across separators. `StringToArrayCastRule` and `StringToRowCastRule` now remember whether a token was quoted, so `""` yields the empty string and `"null"` yields the four-character string rather than SQL NULL.

`StringToMapCastRule` still cannot do either. Its splitter works at the entry level (`k -> v, k2 -> v2`) and hands each entry to `ENTRY_PATTERN` to separate key from value, so by the time `parseValue` runs, the quote state of the individual key or value is gone. `"" -> a` loses the empty key and `k -> "null"` becomes a null value.

Fixing it needs quote-aware key/value splitting inside an entry rather than a regex over the already-stripped text, which is why it was left out of the array and row change.

Start with StringToMapCastRule, especially its entry splitter, ENTRY_PATTERN, and parseValue flow; compare how StringToArrayCastRule and StringToRowCastRule retain quote state. Done means quoted empty keys or values remain empty strings and quoted "null" remains literal text, while unquoted null behavior is preserved.

Written by the indexing model from the issue text.