Handle invalid View Bit Plane colour input

[KairosOps]

Fixes #2503.

View Bit Plane assumed the colour argument always matched one of the RGBA channel names. A pretty URL such as View_Bit_Plane('',7) leaves that argument as an empty string, so the operation ended up reading from an invalid pixel offset instead of reporting a bad argument.

This adds an explicit colour lookup check before scanning the image and returns the same kind of user-facing OperationError that the operation already uses for invalid bit numbers. I also added a regression test using the empty colour argument from the failing URL path.

Checked locally:

• targeted View Bit Plane operation tests
• direct pretty-URL parse/bake check for View_Bit_Plane('',7)
• npx grunt configTests
• node --check src/core/operations/ViewBitPlane.mjs
• node --check tests/operations/tests/Image.mjs
• git diff --check

[GCHQDeveloper581]

This was separately fixed by the implementation of automated option validation in #2625.

However thanks for your contribution, and we hope to see more from you in future.