Normalize plugin type constants

[MyPyDavid]

Summary

• add shared PluginType constants and reuse them across plugin detection and lookups
• align issue provider plugin type slugs with the canonical values
• extend plugin tests to cover detection and issue-provider persistence

Testing

• PYTHONPATH=/workspace/rdmo/testing:/workspace/rdmo DJANGO_SETTINGS_MODULE=testing.config.settings python -m pytest rdmo/config/tests/test_plugins.py

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve issue providers saved with old slug

Project issue providers saved before this change will disappear: this view now filters plugins by project_issue_provider, but prior releases persisted plugin_type='issue_provider' (the base IssueProvider.plugin_type value). Upgrading without migrating existing rows means Plugin.objects.for_context(...) returns no providers for projects, so issue creation/integration flows break until every provider record is re-saved manually. Consider migrating old plugin_type values or accepting both slugs.

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