Refactor Plugin Registration Logic

[davideast]

This PR refactors the plugin registration logic in the dataprompt library.

Previously, PluginManager contained hardcoded logic to inject default plugins (Firestore, Fetch, Scheduler) if they were missing. This created a tight coupling and duplicated logic found in ConfigManager.

This change:

1. Creates src/core/default-plugins.ts which exports getDefaultPlugins() and resolvePlugins().
2. Updates ConfigManager to use this new module to ensure default plugins are present in the configuration.
3. Removes the default plugin injection logic from PluginManager, making it a "dumb" container that simply registers what it is given.
4. Updates unit and integration tests to reflect this change (instantiating PluginManager with explicit defaults where necessary).

This completes the "Refactor Plugin Registration" task outlined in PREVENT_CONFLICTS.md.

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PR created automatically by Jules for task 12190076701982729250 started by @davideast

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