fix: set indexing technique from dataset during update-by-text

[aplio]

Summary

This PR fixes an invalid_param error in the update-by-text API endpoint.
The error occurs when indexing_technique is not provided to the KnowledgeConfig validation, resulting in a Pydantic validation error.

Example of the current error:

curl --location --request POST 'http://127.0.0.1:5001/v1/datasets/UUID_HERE/documents/UUID_HERE/update-by-text' \
--header 'Authorization: Bearer API_KEY_HERE' \
--header 'Content-Type: application/json' \
--data-raw '{"name": "name2","text": "text"}'

Response:

{
    "code": "invalid_param",
    "message": "1 validation error for KnowledgeConfig\nindexing_technique\n  Field required [type=missing, input_value={'name': 'name2', 'text':...4da8-9a39-a78f4108669a'}, input_type=dict]\n    For further information visit https://errors.pydantic.dev/2.9/v/missing",
    "status": 400
}

The indexing_technique parameter is only meant to be set during document creation or initial upload, as shown in the current dataset_service implementation. For update operations, this field should not be required since it's already set and immutable.

This PR fixes the issue by defaulting to the dataset's existing indexing_technique value in the update API endpoint.

resolves #12465

Screenshots

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods