test: replace node-native APIs with ava for testing (copilot-extensions#78)

## Summary

Towards copilot-extensions#73

This PR replaces node-native APIs with `ava` for testing. It does not
tackle the domain of coverage (replacing to `c8`), as I think that would
be better handled in its own PR.

Here's a summary of changes:

- Updates all files in `test/` to use `ava` instead of `node:test`,
using `ava`'s built-in assertion methods
- Revises the snapshot structure to use [ava's snapshot
testing](https://github.com/avajs/ava/blob/main/docs/04-snapshot-testing.md)
rather than the previously generated ones by Node's experimental feature
- Updates `package.json`'s testing scripts to reflect the new `ava`
reality
- Commits the changes from `npm run lint:fix` for style consistency

### Questions for reviewers 👓 

- Snapshots
  - Is the new snapshot structure clear and easy to follow?
- Are we okay with the changes in `package.json` for the
`test:code:update-snapshots` script? (More on [`--update-snapshots`
here](https://github.com/avajs/ava/blob/main/docs/04-snapshot-testing.md).)
- Testing
- Metadata scope: In
copilot-extensions@3871f77,
I removed the `request` and `response` properties from the expectation
to "fit" what [`ava`'s
`.throwsAsync()`](https://github.com/avajs/ava/blob/main/docs/03-assertions.md#throwsasyncthrower-expectation-message)
expects. Do folks prefer that we continue to support the metadata in the
`request-response` pair as a part of the test, or is checking the `name`
and `message` "enough" for what we're looking to check?
- Deep Equality Checks: `ava` has [its version of
`deepEqual()`](https://github.com/avajs/ava/blob/main/docs/03-assertions.md#deepequalactual-expected-message)
which is powered by
[Concordance](https://github.com/concordancejs/concordance). I used it
as a replacement for the previous calls to [Node's
`.assertStrictDeepEqual`](https://nodejs.org/api/assert.html#assertdeepstrictequalactual-expected-message).
Reviewing [Concordance's Compare
Implementation](https://github.com/concordancejs/concordance/blob/main/lib/compare.js),
it would seem as though strict equality is enforced throughout and may
be logically equivalent to what Node had already provided. Despite the
tests passing, I wanted to flag that in the case we wanted to continue
using Node's version still? 💭

Author: francisfuzz

index.d.ts

@@ -116,7 +116,7 @@ export interface CopilotMessage {
   [key: string]: unknown;
 }
 
-export type MessageRole = 'system' | 'user' | 'assistant';
+export type MessageRole = "system" | "user" | "assistant";
 export interface InteropMessage<TRole extends MessageRole = MessageRole> {
   role: TRole;
   content: string;

index.test-d.ts

@@ -168,7 +168,7 @@ export function transformPayloadForOpenAICompatibilityTest(
   expectType<{
     messages: {
       content: string;
-      role: 'system' | 'user' | 'assistant';
+      role: "system" | "user" | "assistant";
       name?: string;
       [key: string]: unknown;
     }[];