Add more tests for Byzantine roubust aggregator

[mina5rovic]

• Adds test coverage for Byzantine robust aggregator
• Covers edge cases and expected behavior
• No aggregator code changes

[JulienVig]

Thanks for the additional tests! fast-check is a cool library I hadn't heard of property-based testing before.
Running the fast check test seems to be a bit flaky, alternating between passing and failing.
Here's what it looks like when it fails (I bumped the number of runs to 2000 to have it fail more frequently)

$ npm -w discojs test -- -t 'bounds the marginal influence of a single Byzantine client'

 FAIL   discojs  discojs/src/aggregator/byzantine.spec.ts > ByzantineRobustAggregator > bounds the marginal influence of a single Byzantine client
Error: Property failed after 1306 tests
{ seed: -459713346, path: "1305:65:63:63:63", endOnFailure: true }
Counterexample: [[Number.NaN,0,0,0]]
Shrunk 4 time(s)
Got AssertionError: expected NaN to be less than or equal to 0.4
    at byzantine-tests-mina5rovic/discojs/src/aggregator/byzantine.spec.ts:209:29
    at processTicksAndRejections (node:internal/process/task_queues:105:5)
    at AsyncProperty.run (file:byzantine-tests-mina5rovic/discojs/node_modules/fast-check/lib/esm/check/property/AsyncProperty.generic.js:46:28)
    at asyncRunIt (file:byzantine-tests-mina5rovic/discojs/node_modules/fast-check/lib/esm/check/runner/Runner.js:33:21)
    at byzantine-tests-mina5rovic/discojs/src/aggregator/byzantine.spec.ts:180:5
    at file:byzantine-tests-mina5rovic/node_modules/@vitest/runner/dist/index.js:915:20

Hint: Enable verbose mode in order to have the list of all failing values encountered during the run
 ❯ discojs/src/aggregator/byzantine.spec.ts:209:29
    207|           const deviation = Math.abs(byz - clean);
    208|           const maxAllowed = 2 * clipRadius / n; // realistic tolerance for extreme inputs
    209|           expect(deviation).toBeLessThanOrEqual(maxAllowed);
       |                             ^
    210|         }
    211|       ),
 ❯ AsyncProperty.run discojs/node_modules/fast-check/lib/esm/check/property/AsyncProperty.generic.js:46:28
 ❯ asyncRunIt discojs/node_modules/fast-check/lib/esm/check/runner/Runner.js:33:21
 ❯ discojs/src/aggregator/byzantine.spec.ts:180:5
 ❯ buildError discojs/node_modules/fast-check/lib/esm/check/runner/utils/RunDetailsFormatter.js:126:15
 ❯ asyncThrowIfFailed discojs/node_modules/fast-check/lib/esm/check/runner/utils/RunDetailsFormatter.js:143:11

[JulienVig]

Suggested change

	"fast-check": "^3"
	"fast-check": "3"

We usually keep the version fixed to avoid unintentional updates and break-downs

[JulienVig]

If you only need to use fast-check for discojs then no need to add it to the whole project. Also this version is different from the one installed in discojs, which one do you want to use?