Fix for concat changes when one diff is there (not possbile to concat)

[MarcelGeo]

• fix migration with sa.text

[Copilot]

Pull request overview

This PR fixes an issue where geodiff.concat_changes fails when there is only one diff to concatenate, and updates an Alembic migration to use the required sa.text() wrapper for raw SQL strings.

Changes:

• Added handling for single-diff checkpoint creation by copying instead of concatenating
• Fixed migration file to wrap raw SQL strings with sa.text() for SQLAlchemy 2.0+ compatibility
• Added test coverage for the single-diff checkpoint scenario

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
server/migrations/community/bd1ec73db389_create_file_diff_table.py	Wrapped raw SQL strings in sa.text() calls and cleaned up trailing whitespace
server/mergin/sync/models.py	Added copy_file import and logic to copy instead of concatenate when there's only one diff
server/mergin/tests/test_public_api_v2.py	Added test case for single-diff checkpoint and increased version count to v33 to support the test

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[coveralls]

Pull Request Test Coverage Report for Build 22310140088

Details

• 15 of 15 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.633%

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Totals
Change from base Build 22227692371:	0.02%
Covered Lines:	8886
Relevant Lines:	9390

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💛 - Coveralls