test(amazonq): fix test failure supplementalContext.test.ts

[Will-ShaoHua]

Problem

same issue #6019 is trying to fix
misuse of sinon.alwaysReturned()

failure
https://github.com/aws/aws-toolkit-vscode/actions/runs/11844970504/job/33009408730
https://github.com/aws/aws-toolkit-vscode/actions/runs/11844970504/job/33009410300

Solution

sinon.returns()

---

License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[justinmk3]

failure seen with latest master: a527f96

 1) crossFileContextUtil
       fetchSupplementalContextForSrc
         for t1 group, should return repomap + opentabs context:

      AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

  assert_1.default.ok(actual.supplementalContextItems.length === 4)

      + expected - actual

      -false
      +true
      
      at Context.<anonymous> (D:\a\aws-toolkit-vscode\aws-toolkit-vscode\packages\amazonq\test\unit\codewhisperer\util\crossFileContextUtil.test.ts:81:20)

[Will-ShaoHua]

checking