feat(testing): implement template database cloning for MySQL

[CodingAnarchy]

Summary

This PR implements template database cloning for sqlx::test to significantly speed up test runs when using migrations. Instead of running migrations for every test, a template database is created once and cloned for each test.

Before (slow)

For each test:
  CREATE DATABASE test_xyz
  → Run all migrations (slow!)
  → Load fixtures
  → Run test
  → DROP DATABASE

After (fast)

Once per migration set:
  CREATE DATABASE _sqlx_template_<hash>
  → Run all migrations

For each test:
  Clone template to test_xyz (fast!)
  → Load fixtures
  → Run test
  → DROP DATABASE

Implementation Details

Core Infrastructure (sqlx-core/src/testing/mod.rs)

• Add migrations_hash() - computes SHA256 hash of all migration checksums for template naming
• Add template_db_name() - generates template database name from hash
• Extend TestContext with from_template: bool field
• Modify setup_test_db() to skip migrations when cloned from template

MySQL Implementation (sqlx-mysql/src/testing/mod.rs)

• Add _sqlx_test_templates tracking table for template metadata
• Use GET_LOCK() for synchronization during template creation
• Clone via mysqldump/mysql commands (fast) with in-process SQL fallback
• Handle existing template databases from previous runs (idempotent)
• Compatible with both MySQL and MariaDB

Configuration

• Opt-out: Set SQLX_TEST_NO_TEMPLATE=1 to disable template cloning
• Template names include migration hash, so changing migrations automatically creates new templates

Test Plan

• Unit tests for migrations_hash() and template_db_name() functions
• Integration tests verifying template creation and reuse
• Integration tests verifying fixtures work with cloned templates
• CI passes on all MySQL versions (8.4, 8.0, 5.7)
• CI passes on all MariaDB versions (verylatest, 11.8, 11.4, 10.11, 10.6)
• CI passes on all runtime/TLS combinations

Future Work

• PostgreSQL implementation using native CREATE DATABASE ... TEMPLATE (follow-up PR)
• SQLite implementation using file copy (follow-up PR)

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