VertexAI: update test cases to use full name of mock response files

[tanzimfh]

This will allow these files to be scanned by a coverage tool to be added to https://github.com/firebaseextended/vertexai-sdk-test-data.

[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add the 'no-changelog' label to the PR to silence this warning.)

Generated by 🚫 Danger

[google-oss-bot]

Coverage Report ¹

Affected Products

No changes between base commit (1b49171) and merge commit (7f474f4).

Test Logs

• Base (1b49171): https://github.com/firebase/firebase-android-sdk/actions/runs/10111230899
• Merge (7f474f4): https://github.com/firebase/firebase-android-sdk/actions/runs/10113528988

1. https://storage.googleapis.com/firebase-sdk-metric-reports/3Tw66gNhmi.html

[github-actions]

Unit Test Results

    4 files  +    4      4 suites  +4   3s ⏱️ +3s
  50 tests +  50    50 ✔️ +  50  0 💤 ±0  0 ❌ ±0 
100 runs  +100  100 ✔️ +100  0 💤 ±0  0 ❌ ±0 

Results for commit dcae90c. ± Comparison against base commit 1b49171.

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (1b49171) and merge commit (7f474f4).

Test Logs

• Base (1b49171): https://github.com/firebase/firebase-android-sdk/actions/runs/10111230899
• Merge (7f474f4): https://github.com/firebase/firebase-android-sdk/actions/runs/10113528988

1. https://storage.googleapis.com/firebase-sdk-metric-reports/mCR962Nv3l.html