GL_ARB_bindless_texture input sampler for fragment shader must have the qualifier "flat"

As defined in "Section 4.3.4, Inputs, p. 34" of https://registry.khronos.org/OpenGL/extensions/ARB/ARB_bindless_texture.txt

Fragment shader inputs that are signed or unsigned integers,
integer vectors, or any double-precision floating- point type, or any
sampler or image type must be qualified with the interpolation qualifier
"flat".

Current implementation violates this rule.

Start with Section 4.3.4 of the ARB_bindless_texture specification and trace glslang's validation of fragment shader inputs involving sampler or image types. Confirm the current behavior with a minimal shader, then add or update coverage so these inputs require the flat interpolation qualifier.

Written by the indexing model from the issue text.