feat(module/mysql): Add getR2dbcUrl() to MySQLR2DBCDatabaseContainer

[papa-piton-2256]

Motivation

This PR implements the static getR2dbcUrl() helper method for the MySQL module, as requested by @eddumelendez in issue #8797.

Currently, users working with R2DBC drivers have to manually construct connection strings, which is error-prone and verbose. This helper provides a standardized way to obtain a valid R2DBC URL from a running MySQLContainer.

Changes

• Added public static String getR2dbcUrl(MySQLContainer<?> container) to MySQLR2DBCDatabaseContainer.
• Returns a formatted string: r2dbc:mysql://<user>:<password>@<host>:<port>/<db>.

Verification

• Ran ./gradlew :testcontainers-mysql:test locally.
• Verified that the code style complies with Spotless.

Fixes #8797 Supersedes #9569 (inactive)