fix: normalize reader output

[abnegate]

Summary

• Normalize Appwrite column list models at the reader boundary.
• Preserve the reader plain-array contract for polymorphic column responses.
• Add a public-path regression using the current SDK list model.

Changes

• Read serialized columns from the SDK ColumnList model instead of serializing each raw entry independently.
• Cover API::listColumns() with real Database and Table resources and the locked Appwrite SDK 26.1.0 model.

Scope in the DAT-2310 release train

This is a consumer-boundary compatibility leaf, not the production empty-object fix by itself. It ensures the migration reader keeps returning list<array> when the SDK list model materializes polymorphic column entries as already-normalized arrays. DAT-2310 still requires the coordinated HTTP, database, Mongo, server-ce, and cache fixes before an empty JSON object can be called preserved end to end.

Test plan

• Focused regression observed failing before the production change and passing afterward
• Composer validation and advisory audit
• Formatting and PHPStan
• Unit suite: 65 tests, 391 assertions
• Isolated Docker full suite: 87 tests, 475 assertions
• Fresh main merge: no conflicts; 83 unit tests, 457 assertions

Refs DAT-2310

[greptile-apps]

Greptile Summary

The PR moves Appwrite column normalization to the SDK list-model boundary while preserving the reader’s plain-array output contract.

• Serializes the complete ColumnList response before extracting its columns.
• Adds a regression test using real database and table resources with the SDK list model.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/Migration/Sources/Appwrite/Reader/API.php	Normalizes the complete SDK ColumnList response and returns its serialized columns.
tests/Migration/Unit/Sources/AppwriteReaderApiTest.php	Adds coverage confirming that API::listColumns() returns the expected list of plain column arrays.

_{Reviews (2): Last reviewed commit: "Merge branch 'main' into fix/reader-norm..." | Re-trigger Greptile}

[abnegate]

Latest-compatible dependency audit at exact head c397bb817b9956351875422971bcd03a327f8b34:

• appwrite/appwrite: 26.1.0, the newest release inside this package's declared ^26.0 contract (27.1.0 is the registry latest but is outside the declared major)
• utopia-php/database: 7.1.3, updated with utopia-php/storage 4.0.3 and utopia-php/validators 0.4.2 because those constraints move together
• tests/Migration/Unit: 65 tests, 391 assertions, green
• focused AppwriteReaderApiTest: 1 test, 5 assertions, green

This was a disposable compatibility resolution only; no VCS pin or lock-file change is being added to this PR. The full local suite was not counted as a pass because its NHost and Supabase E2E services were not running (8 connection errors); the PR's own current-head CI remains the authoritative full gate.