Incorrect interpolation of single quote for multi-value variable

### Summary
The plugin's automatic quoting for multi-value and "Include All" variables uses the wrong escape sequence for embedded single quotes. It doubles the quote (`''`), which is ANSI SQL syntax. BigQuery does not support this — [instead it supports a backslash escape](https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/lexical#quoted_literals) (`\'`).

### Steps to reproduce
1. Create a multi-value (or Include All) query variable whose options include a value containing a single quote character. (eg. `O'Reilly`)
2. Reference it in a query: `AND x IN (${my_var})`.
3. Select that value (or select "All").

### Expected
The plugin escapes the apostrophe using BigQuery's syntax:
`delivery_provider IN ('O\'Reilly')`

### Actual
The plugin doubles the quote instead, producing invalid BigQuery syntax:
`delivery_provider IN ('O''Reilly')`

which BigQuery parses as two adjacent string literals and rejects:

Error 400: Syntax error: concatenated string literals must be separated by whitespace or comments at [line:col], invalidQuery

### Environment
- grafana-bigquery-datasource version: 3.1.5
- Grafana version: 11.6.1

Start by locating the plugin's automatic quoting and interpolation logic for multi-value and “Include All” variables. Reproduce the issue with a value such as O'Reilly in a BigQuery query, then verify that the generated IN expression uses BigQuery's backslash escape and no longer produces the reported syntax error.

Written by the indexing model from the issue text.