Add getR2dbcUrl() helper to PostgreSQL, MySQL, and MariaDB R2DBC containers

[PreAgile]

Summary

• Add static getR2dbcUrl(container) helpers for PostgreSQL, MySQL, and MariaDB R2DBC containers
• Update both modern and compatibility package variants
• Add tests covering generated URL shape and actual R2DBC connectivity

Problem

R2DBC support already exists via getOptions(container), but there is no simple URL helper similar to getJdbcUrl(). Users working with reactive stacks often need a plain R2DBC URL and currently have to construct it manually.

Design

• Scope is limited to PostgreSQL, MySQL, and MariaDB
• MSSQL is intentionally deferred due to the existing database-name limitation in its R2DBC support
• The helper intentionally excludes credentials, mirroring getJdbcUrl()
• The change follows the issue discussion by adding static helper methods on concrete implementations rather than changing a shared base API

Tests

• Added testGetR2dbcUrl() to modern and compatibility test classes
• Each test sets a custom database name, username, and password
• Each test asserts the generated URL contains the custom database name
• Each test creates a ConnectionFactory from the generated URL plus credentials and runs a query
• CI passes

Closes #8797