ArrayDecoder doesn't handle escaped backslashes in quoted PostgreSQL array elements

## Problem

The `decode_quoted_elem` function in `packages/elixir-client/lib/electric/client/ecto_adapter/array_decoder.ex` does not correctly handle escaped backslashes in quoted PostgreSQL array elements.

PostgreSQL array format requires `\\` (escaped backslash) in quoted strings to be decoded as a single backslash `\`, similar to how `\"` decodes to a quote. The current implementation only handles escaped quotes but treats backslashes as regular UTF8 characters, causing arrays containing literal backslashes to be decoded incorrectly—each `\\` results in two backslashes instead of one.

## Example

A PostgreSQL array like `{"a\\\\b"}` should decode to `["a\\b"]`, but currently decodes to `["a\\\\b"]`.

## Context

This issue was identified during review of PR #3699 but is pre-existing in the codebase and not introduced by that PR.

## References

- PR: https://github.com/electric-sql/electric/pull/3699
- Comment: https://github.com/electric-sql/electric/pull/3699#discussion_r2690724545
- Reported by: @robacourt