storcon: Return only generation when increment

[ephemeralsad]

Problem

The increment_generation logic currently returns the entire TenantShardPersistence object, even though only the generation field is needed.

Summary of Changes

• Updated the query to return only the generation value instead of the full TenantShardPersistence struct.

[github-actions]

8481 tests run: 7900 passed, 0 failed, 581 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_sharding_split_failures[failure2]: debug-x86-64-without-lfc

Postgres 14

• test_versions_mismatch[combination_nnnnn]: release-arm64-with-lfc
• test_sharding_split_failures[failure9]: release-arm64-with-lfc

Code coverage* (full report)

• functions: 32.1% (9024 of 28104 functions)
• lines: 48.4% (80219 of 165758 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
befab6b at 2025-06-14T11:56:37.542Z :recycle:}

[problame]

LGTM, although I'm not a diesel expert