[Relax][ONNX] Fix shape/dynamic restrictions for Squeeze/Unsqueeze and Slice (#18955)

## Summary

Relates to #18945.

This PR improves ONNX frontend handling for dynamic
`Unsqueeze`/`Squeeze`/`Slice`, tightens validation paths, and adds
targeted structural/negative regression tests.

- Refactor constant-path `Unsqueeze` lowering to use a single `reshape`
based on computed target shape.
- Remove scalar-specific branching and repeated `expand_dims` in the
constant path.
- Add/keep structural helper usage in ONNX frontend tests for Relax
call-op checks.
- Add regression coverage for scalar-input `Unsqueeze`.

## Changes

- Add dynamic-axes conversion paths for `Unsqueeze` and `Squeeze`:
  - infer output shape via runtime shape-tensor construction
- lower to `relax.reshape` with validated shape rank/length assumptions
- Improve `Slice` conversion robustness:
  - support dynamic parameter forms with stricter rank/length validation
  - reject invalid zero-step inputs when statically known
  - fix docstring wording (`Splice` -> `Slice`)
- Strengthen ONNX frontend tests:
  - negative test for duplicate `Unsqueeze` axes
- structural IR check for dynamic `Slice` (`relax.dynamic_strided_slice`
present, `relax.strided_slice` absent)
  - negative test for zero-step `Slice`
- Refactor constant-path `Unsqueeze` scalar handling:
- replace scalar special-casing + repeated `expand_dims` with one
target-shape `reshape`
  - add scalar-input regression test
- Restore shared test helper used by structural Relax call-op checks.

## validation

- `ruff check`: passed
- `pre-commit --files`: passed
- `pytest`: 8 passed

Author: Aharrypotter