Add Summarization operation type support to the echoai test provider
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## Description
The echoai mock provider (`Drupal\ai_test\Plugin\AiProvider\EchoProvider`, in the `ai_test` submodule) implements several operation types so that downstream modules can kernel-test their integrations against a deterministic provider. It does not yet implement the **Summarization** operation type.
## Acceptance criteria
* `echoai__ai` reports support for the `summarize` operation type and is selectable for it.
* Calling `summarize()` with text `Foo` returns a `SummarizationOutput` whose normalized value is `Summary of: Foo`.
* Consumers (e.g. the `ai_integration_eca` Summarization action) can be kernel-tested against `echoai__ai` without a real provider.
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## Testing instructions *(added by implementor before review)*
1. Enable `ai` and `ai_test`.
2. In a kernel test, load the `echoai__ai` provider and call `->summarize('Foo', $model_id)`; assert `getNormalized()` is `Summary of: Foo`.
3. Confirm the existing echo operations (chat, etc.) are unchanged.
## Related issues
* /relate ai_integration_eca#3585009
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