fix(connector): preserve SQL Server composite primary key order

[zwang28]

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What's changed and what's your intention?

• Preserve SQL Server CDC auto-discovered column order and composite primary-key order by sorting catalog rows by their ordinal positions.
• Add regression coverage for schema derivation and batched snapshot reads when physical column order differs from primary-key order.
• Related to bug(connector): MySQL CDC schema discovery does not preserve composite primary key order #26846.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

• My PR needs documentation updates.

Release note

Summary by CodeRabbit

• Bug Fixes

  • Improved SQL Server CDC handling for tables with composite primary keys.
  • Preserved the source table’s column and primary-key ordering during replication.
  • Improved consistency when mapping and querying replicated rows from composite-key tables.

• Tests

  • Added coverage for composite primary-key tables and CDC replication scenarios in SQL Server.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: d9bc6335-d502-44c1-9e40-c3eb5cedf6a7

📥 Commits

Reviewing files that changed from the base of the PR and between c4a578b and 2e1e1a5.

📒 Files selected for processing (3)

• e2e_test/source_inline/cdc/sql_server/sql_server_cdc.slt.serial
• e2e_test/source_inline/cdc/sql_server/sql_server_cdc_prepare.sql
• src/connector/src/source/cdc/external/sql_server.rs

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

SQL Server CDC metadata queries now preserve table and primary-key order. End-to-end tests add a composite-key source table and verify its mapped schema and replicated rows.

Changes

SQL Server CDC ordering

Layer / File(s)	Summary
Preserve SQL Server metadata ordering src/connector/src/source/cdc/external/sql_server.rs	The column metadata query orders results by ORDINAL_POSITION. The primary-key query orders results by kcu.ORDINAL_POSITION.
Validate composite-key CDC e2e_test/source_inline/cdc/sql_server/sql_server_cdc_prepare.sql, e2e_test/source_inline/cdc/sql_server/sql_server_cdc.slt.serial	The test setup creates a CDC-enabled table with a three-column primary key and seed rows. The test verifies mapped column order, primary-key order, distribution-key order, and replicated rows.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 2e1e1

The change preserves SQL Server CDC column and composite-key ordering and adds regression coverage, with no identified merge-blocking risk.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (2 skipped: 2 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving SQL Server composite primary key order in the connector.
Description check	✅ Passed	The description explains the fix, implementation approach, regression coverage, and related issue. It follows the required template and completes the relevant checklist items. The template requests a …
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files. (2 skipped: 2 unsupported.)

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[chenzl25]

LGTM

[github-actions]

✅ Cherry-pick PRs (or issues if encountered conflicts) have been created successfully to all target branches.