Implement `MigrateSqlGetMigrationPluginManagerRector`
### Problem/Motivation
Replace deprecated Sql::getMigrationPluginManager() calls with direct $this->migrationPluginManager property access. The method is deprecated in drupal:9.5.0 and removed in drupal:11.0.0.
### Proposed resolution
Handled automatically by drupal-rector via the `MigrateSqlGetMigrationPluginManagerRector` rule.
### How this rule was built
This rule was produced with an AI-assisted workflow in which every step was verified manually by @bbrala. Its logic started from the matching drupal-digests entry, which was adapted to drupal-rector's conventions — a dedicated rule or an existing upstream rule configured into the Drupal 12 set — and covered with before/after test fixtures. Backward-compatibility was assessed and version-gated: safe rewrites are applied directly, while unsafe ones are wrapped via `DeprecationHelper` or moved to an opt-in breaking set. The rule was then live-tested against a set of contributed modules; where no contrib call site could be found, a static code sample was written in a test module to exercise it. A final automated QA pass covered type-guard, fixture coverage, BC-decision, registration and idempotency checks.
### References
- Change record(s): [CR-3442785](https://www.drupal.org/node/3442785)
- Related issue(s): [#3439369](https://www.drupal.org/i/3439369)
### Work items
- Commit(s): [7cd65aa7a9](https://github.com/palantirnet/drupal-rector/commit/7cd65aa7a9ba)
### Credit
- Implemented by: @bbrala
- Reviewed / helped by: @mglaman
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*Filed retroactively as part of the Drupal 12 rector coverage effort. Closed as fixed; credit to be applied to the contributors listed above.*
*AI was used extensively to build this rule and to file this issue. See [Dries Buytaert: AI-generated Rector rules for Drupal](https://dri.es/ai-generated-rector-rules-for-drupal) and [Björn Brala: Drupal Rector rules are config, not code. Mostly.](https://www.linkedin.com/pulse/drupal-rector-rules-config-code-mostly-bj%C3%B6rn-brala-vfpwe/).*
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