[MLIR][Test] Extend coverage of unrealized_conversion_cast operation

Adds a suite of IR tests for the unrealized_conversion_cast operation,
covering a wide range of type conversion scenarios and edge cases:

- Basic functionality
- Identity casts (input and output types are the same)
- Type system boundaries (e.g., shaped, tuple, and custom types)
- Pass integration and reconciliation behavior
- Location and attribute preservation
- Structural, usage pattern and compilation-time stress (e.g.,
  fan-out, deep chains)
- Integration with control flow and region-based operations
- Memory types and dimensionality boundaries (e.g., dynamic shapes,
  unranked tensors)
- Canonicalization interaction (e.g., with constant folding, CSE, DCE)
- Complex and nested type patterns
- Edge cases and boundary conditions (e.g., bitwidths, scalable vectors)

These tests help guard against regressions and provide a robust
reference for dialect authors using unrealized_conversion_cast in
type conversion scenarios across dialect boundaries.

Author: peymanbr