fix(cli): Don't detect column renames for live columns that still exist in the target - #5920

generateTableMigration matches source and target columns by effectiveFieldName, which falls back to the column name when fieldName is null. A schema read from a live database never has field names, so when a model maps fields to each other's column names (first: String, column=second / second: String, column=first), each live column was matched to the wrong field and the diff reported a rename cycle (first → second, second → first) for a table that is already in sync. The generated SQL can't run either, because each target name still exists when its rename runs.

Source columns without a stored fieldName are now only considered for a rename when their name no longer exists in the target. If the target still has a column with that name, it's the same physical column, as the issue suggests. Columns with a stored fieldName are matched exactly as before.

Tests in column_rename_test.dart:

• the swapped-columns case from the issue (live schema without field names) now produces no migration actions;
• a definition without field names whose column was renamed in the target is still detected as a rename, so the legacy fallback keeps working.

The swapped-columns test fails without this change. dart test test/database in tools/serverpod_cli passes (773 tests).

Fixes #5915

Pre-launch Checklist

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• I added new tests to check the change I am making.
• All existing and new tests are passing.
• Any breaking changes are documented below.

Breaking changes

None.