Bound handleize stack allocation

[Copilot]

handleize sized its stack buffer from user-controlled input length, allowing large values to terminate the process with an uncatchable stack overflow.

• Allocation safety

  • Use a fixed 512-character stack buffer.
  • Let ValueStringBuilder expand into pooled heap storage for larger output.

• Regression coverage

  • Add coverage for a 377,000-character input.

var result = new ValueStringBuilder(stackalloc char[512]);

• Fixes Uncatchable stack overflow in Fluid's handleize filter from an unbounded stackalloc #996