Preserve output names for returned input aliases

[TANGBUDU]

Fixes #2714.

When a graph input is returned through a local alias, use the alias name for the inserted Identity output instead of the generic return_val name. Directly returning an unchanged graph input keeps the existing naming behavior. Generated names still go through the existing uniqueness allocator.

The method audit found a missed case: Y = X; X = op.Neg(X); return Y returned the original input under the name X, not Y. Looking up the resolved ONNX name in the mutable Python symbol table incorrectly classified the value after X was rebound. The converter now checks the resolved IR value directly.

Regression coverage includes input-name rebinding, collisions with an existing input name, duplicate alias returns, ONNX checker validation, and runtime output parity. The rebinding regression fails before the follow-up fix ('X' != 'Y') and passes afterward.

Tests (2026-09-07):

• python -m pytest onnxscript/_internal/converter_test.py -q: 56 passed, 1 skipped, 1 xfailed, 3 xpassed; 160 subtests passed, both with ONNX 1.22.0 / ONNX Runtime 1.29.0 and with ONNX 1.18.0 / ONNX Runtime 1.23.0 / ONNX IR 0.1.16.
• lintrunner -a: no lint issues.
• git diff --check: clean.

AI assistance was used for the source audit, patch preparation, and validation. The existing expected-failure / unexpected-pass results are reported above, not counted as ordinary passes.

[TANGBUDU]

Justin Chu (@justinchuby) gentle ping on this PR when you have a chance. It addresses #2714 and is ready for review. Happy to address any feedback — thanks!