test(teeny-request): add multi-byte UTF-8 Content-Length unit test (#9763)

## Description

Adds a multi-byte UTF-8 string `Content-Length` unit test for
`bunFetch`.

## Impact

Verifies byte-accurate `Content-Length` headers for non-ASCII string
request bodies.

## Changes

- Follow up on review feedback in
#9748
(`b/578941498`).
- Add a unit test case in `core/packages/teeny-request/test/index.ts`
sending a multi-byte UTF-8 string body (`'hello 🌍'`, 8 UTF-16 code units
/ 10 UTF-8 bytes) via `bunFetch` and asserting that `Content-Length` is
`'10'`.

## Testing

- Ran `core/packages/teeny-request` unit tests 5 consecutive times on
Node.js (`pnpm --dir core/packages/teeny-request test`) and 5
consecutive times on Bun (`JS_RUNTIME=bun MOCHA_PARALLEL=false bun --bun
run --cwd core/packages/teeny-request test`).
- Verified `GIT_DIFF_ARG="upstream/main...HEAD -- :!packages" node
./bin/linter.mjs --strict` and `ci/run_conditional_tests.sh --strict`
pass.

## Alternatives

- Relying solely on the existing ASCII `'hello'` test was considered and
rejected because ASCII strings have identical character length and UTF-8
byte length (`5`), which would not catch regressions that use
`body.length` instead of `Buffer.byteLength(body)`.
- Not merging this PR was considered, which would leave multi-byte UTF-8
string `Content-Length` calculation in `bunFetch` without regression
coverage.

Author: danieljbruce

core/packages/teeny-request/test/index.ts

@@ -707,6 +707,31 @@ describe('teeny', () => {
         nock.disableNetConnect();
       }
 
+      const multibyteContentLengthScope = nock(uri)
+        .post('/content-length-utf8', 'hello 🌍')
+        .matchHeader('content-length', '10')
+        .reply(200, 'ok');
+      await new Promise<void>((resolve, reject) => {
+        teenyRequest(
+          {
+            uri: `${uri}/content-length-utf8`,
+            method: 'POST',
+            body: 'hello 🌍',
+          },
+          (err, res, body) => {
+            if (err) return reject(err);
+            try {
+              assert.strictEqual(res.statusCode, 200);
+              assert.strictEqual(body, 'ok');
+              multibyteContentLengthScope.done();
+              resolve();
+            } catch (e) {
+              reject(e);
+            }
+          },
+        );
+      });
+
       const contentLengthScope = nock(uri)
         .post('/content-length', 'hello')
         .matchHeader('content-length', '5')