JSON parser tolerates illegal commas

**Describe the bug**

The JSON tape decoder currently accepts invalid JSON such as:
```json
{, "a": 1, ,,, , "b":2,, }
[,, 1, ,,, , 2, ]
```

**To Reproduce**

Try parsing the above JSON.

**Expected behavior**

Leading, trailing, and repeated commas should produce a parsing error.

From the [JSON spec](https://www.json.org/json-en.html):
> An object is an unordered set of name/value pairs. An object begins with `{` left brace and ends with `}` right brace. Each name is followed by `:` colon and the name/value pairs are separated by `,` comma.
>
> An array is an ordered collection of values. An array begins with `[` left bracket and ends with `]` right bracket. Values are separated by `,` comma.
>

**Additional context**

The tape decoder lacks a `DecoderState::Comma` enum variant. Instead `DecoderState::Object` and `DecoderState::List` both rely on the following (overly permissive) string search:
```rust
iter.advance_until(|b| !json_whitespace(b) && b != b',');
```

The tape decoder's DecoderState::Object and DecoderState::List handling is the entry point; inspect their comma-skipping logic first. Add coverage for leading, trailing, and repeated commas in the JSON examples and verify each now produces a parsing error.

Written by the indexing model from the issue text.