Fix select_as aliases in combined selects

[boopy253]

Fixes #3174.

Root cause

A column-level select_as transform changes the projection from a column expression into a cast expression. Combined selects prefix the first-entity projection aliases, but can only infer an unaliased projection name from a plain column or an enum cast. As a result, find_also_related and the other combined-select paths panic before executing SQL.

Fix

Preserve the column logical name as the projection alias whenever select_as produces a transformed expression. Plain columns and the existing enum path retain their previous SQL shape. Apply the same behavior to default entity projections and QuerySelect::column, and mirror it in sea-orm-sync.

Verification

• Reproduced the original panic with the reported citext mapping in an isolated crate.
• Executed the fixed query end to end against PostgreSQL 18 with the citext extension, including model decoding and case-insensitive filtering.
• Added regression coverage for default and select_only combined selects.
• Passed cargo test --workspace, the full PostgreSQL integration matrix, the sea-orm-sync SQLite matrix, feature checks, Clippy with warnings denied, rustfmt, and Taplo formatting.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-05T17:54:56.347620Z	df8239f	New commits

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[Huliiiiii]

Thanks!