refactor: Migrate document reindexing to UploadDocumentAdapter with unified indexing pipeline

[AnishSarkar22]

Description

• Converted index_uploaded_file function into UploadDocumentAdapter class with index() and reindex() public methods, both routing through IndexingPipelineService.
• Replaced legacy manual reindex logic in document_reindex_tasks.py with a single adapter.reindex() call.
• Added dynamic character budget for document formatting based on model context window, with token-limit overflow handling in llm_router_service.
• Expanded integration test suite from 4 to 11 tests covering reindex content/hash updates, chunk replacement, empty-markdown guard, and error message matching.

Motivation and Context

FIX #

Screenshots

API Changes

• This PR includes API changes

Change Type

• Bug fix
• New feature
• Performance improvement
• Refactoring
• Documentation
• Dependency/Build system
• Breaking change
• Other (specify):

Testing Performed

• Tested locally
• Manual/QA verification

Checklist

• Follows project coding standards and conventions
• Documentation updated as needed
• Dependencies updated as needed
• No lint/build errors or new warnings
• All relevant tests are passing

High-level PR Summary

This PR refactors the document indexing logic by converting the standalone index_uploaded_file function into an UploadDocumentAdapter class with separate index() and reindex() methods. The new reindex() method replaces the legacy manual reindexing logic in the Celery task handler, routing both workflows through the unified IndexingPipelineService. The changes also include comprehensive test coverage expansion from 4 to 11 tests, covering various reindexing scenarios including content hash updates, chunk replacement, empty markdown guards, and error handling.

⏱️ Estimated Review Time: 15-30 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/app/indexing_pipeline/adapters/file_upload_adapter.py
2	surfsense_backend/tests/integration/indexing_pipeline/adapters/test_file_upload_adapter.py
3	surfsense_backend/app/tasks/celery_tasks/document_reindex_tasks.py
4	surfsense_backend/app/tasks/document_processors/file_processors.py

[vercel]

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[recurseml]

Review by RecurseML

🔍 Review performed on 1e4b8d3..ce82807

✨ No bugs found, your code is sparkling clean

✅ Files analyzed, no issues (4)

• surfsense_backend/app/indexing_pipeline/adapters/file_upload_adapter.py
• surfsense_backend/app/tasks/celery_tasks/document_reindex_tasks.py
• surfsense_backend/app/tasks/document_processors/file_processors.py
• surfsense_backend/tests/integration/indexing_pipeline/adapters/test_file_upload_adapter.py

[recurseml]

Review by RecurseML

🔍 Review performed on ce82807..b2bf00e

✨ No bugs found, your code is sparkling clean

✅ Files analyzed, no issues (1)

• surfsense_backend/tests/integration/indexing_pipeline/adapters/test_file_upload_adapter.py