formats mappings adjustments (breaking batch)

While [working on documenting the different format that can be used in OAS](https://github.com/OAI/OpenAPI-Specification/pull/3167) I realized Kiota was not mapping some formats properly:

- [ ] "decimal" should only be a child of string type as the serialization process will trim trailing zeros for numbers, loosing precision. => map string:decimal to a string (decimal type) on the wire, and number:decimal to a number (double) on the wire. (BREAKING)
- [ ] "base64" and "base64url" are different in their serialization and deserialization logic, and should ultimately map to stream. (BREAKING)
- [ ] should we add support for char? (BREAKING)
- [ ] type array with uniqueItems: true (items) should map to a hashset

The issue names no files, tests, or entry points. Start by locating Kiota's format-mapping logic and related tests, then trace the listed decimal, base64, char, and uniqueItems cases through generation and serialization. Done means each agreed mapping is covered by tests, including the documented breaking behavior.

Written by the indexing model from the issue text.