fix(chunk): fix unexisting get_document_chunk call

[tibo-pdn]

Description

Fixes a 500 error on GET /v1/documents/{document_id}/chunks/{chunk_id} caused by a call to a non-existent method get_document_chunk on the document manager. The correct method is get_document_chunks (plural).
The argument order was also adjusted to match the method's signature.

Resolves #781.

Overview

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Area

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• Core / Global project settings
• API
• Playground / Web UI
• DevOps
• Docusaurus / Documentation
• Other (specify below)

Type of change

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• New feature
• Bugfix
• Enhancement (improvement of an existing feature)
• Refactor (change that neither fixes a bug nor modifies behavior)
• Documentation
• Tests
• Performance improvement
• Chore / Maintenance (change to the build process or auxiliary tools, dependencies update, etc.)

Definition of Done / Technical changes

Please provide the Definition of Done (DoD) criteria that apply to this PR.

• get_document_chunk renamed to get_document_chunks in api/endpoints/documents.py:250
• user_id argument repositioned to match the correct method signature

Screenshots / Demo (if applicable)

For more screenshots, please refer to #781.

Breaking changes

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• No breaking changes
• This PR contains breaking changes (explain below)

Quality assurance & Review readiness

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This section helps ensure the PR is ready for review, safe to merge, and deployable. If any items are left unchecked, please add a brief explanation for context.

Documentation

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• No documentation needed
• README / Markdown files updated
• API documentation updated (Swagger / Redoc)
• Docstrings updated
• Inline code comments added where needed

Tests

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• No tests added (explain below)
• Unit tests added
• Integration tests added
• Functional tests added
• End-to-end tests added
• Performance tests added
• Existing tests updated

The fix is a one-line method name correction - no new logic introduced. Existing integration tests covering this endpoint should be sufficient. Also, tests are completely nder clean architecture migration. This branch is just a quick fix.

Code Standards

• Code follows project conventions and architecture
• No unused imports, variables, functions, or classes
• No debug logs or commented-out code left
• No secrets or environment variables committed in clear text
• Code is linted and formatted using the project tools (ruff, etc.)

Git & Process Standards

• PR title follows Conventional Commits
• PR is correctly labeled
• PR is linked to relevant issue(s) / project(s)
• Author is assigned to the PR
• At least one reviewer has been requested

Deployment Notes

• No special deployment steps required
• Requires database migration (see "Database migration" section)
• Requires new or updated environment variables (explain below)
• Requires other special deployment steps (explain below)

Database migration

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• No database migration required
• This PR requires a database migration (see checklist below)

Please confirm that the following steps have been completed for the database migration:

• Migration script added to api/alembic/versions/ folder
• Migration upgrade tested locally
• Migration downgrade tested locally
• Migration documented (if applicable)

Reviewer Focus

Single-line fix in api/endpoints/documents.py:250 — verify the correct method name and that the argument order matches get_document_chunks's signature.