fix: jsonata $base64decode/$base64encode in Web Worker

[min23asdw]

Summary

Fixes $base64decode and $base64encode JSONata functions which throw Cannot read properties of undefined (reading 'from') when used in the browser.

Root cause: The jsonata library checks typeof window !== 'undefined' to choose between browser atob/btoa and Node.js Buffer.from. CyberChef runs operations in a Web Worker where window is undefined, so it falls back to global.Buffer — which also doesn't exist in workers.

Fix: Register custom $base64decode/$base64encode functions on the expression object using atob()/btoa(), which are available in both browser window and Web Worker scopes.

Closes #2063

Changes

File	Change
src/core/operations/Jsonata.mjs	Register base64 function overrides before evaluate()
tests/operations/tests/Jsonata.mjs	Add 2 test cases for $base64decode and $base64encode

Test plan

• npx grunt lint — passes
• npm test — 1893/1893 passing (Node 18)
• Verified error reproduces at https://gchq.github.io/CyberChef/#recipe=Jsonata_Query('$base64decode(%22SGVsbG8gV29ybGQh%22)')&input=e30

▎ This pull request was created with the assistance of Claude and Antigravity

AI usage: I used AI (Claude and Antigravity) to suggest ideas on where it might be broken and to help write the tests.
I then searched online to verify whether those suggestions were correct,
confirmed it myself,
and then actually ran the tests locally.

[GCHQDeveloper581]

Could you also add a (single) test in tests/browser/02_ops.js to test this in a browser context.

[min23asdw]

Add browser test @GCHQDeveloper581 I do it right?. this

[GCHQDeveloper581]

Looks good apart from the linting error, though (because of that) I haven't seen whether the test passes or fails yet!

Also - on PR #2270 you indicated that you'd used the assistance of AI tools. Is this also the case for your other PRs (including this one?)

[min23asdw]

I use AI to add Add browser test (antigravity) and i forgot to run lint check on last commit .... i feel bad . my bad sorry

[min23asdw]

Before

Fix:
/#recipe=Jsonata_Query('$base64decode("SGVsbG8gV29ybGQh")')&input=e30

/#recipe=Jsonata_Query('$base64encode("Hello%20World!")')&input=e30

prove that. It Look good on http://localhost/

@GCHQDeveloper581

[C85297]

LGTM from a code standpoint but from an AI standpoint we will have to confer

[min23asdw]

I won't be sad if the fix doesn't come from my PR ..... I just want to see it fixed.
If it helps feel free to cherry-pick the code.

• AI usage: I used AI (Antigravity) to suggest ideas on where it might be broken and to help write the tests.

I then searched online to verify whether those suggestions were correct,
confirmed it myself,
and then actually ran the tests locally.

[GCHQDeveloper581]

Looks good.

Thank you for your contribution.