fix(developer-knowledge): make fallback search response iterable

[aniruddhaadak80]

Description

Fixes #14609

The Developer Knowledge test fallback represented SearchDocumentChunksResponse as an object with a results attribute but did not make that object iterable. The sample intentionally consumes the response as a pager, so environments without the preview client failed before the sample could run. The fallback now yields its stored chunks, matching the real response behavior without changing production code or dependencies.

Validation:

• python -m pytest developer-knowledge -q — 4 passed
• Pre-fix run — 3 passed, 1 failed with TypeError: 'SearchDocumentChunksResponse' object is not iterable
• python -m ruff check developer-knowledge/conftest.py — passed
• python -m black --check developer-knowledge/conftest.py — passed
• Live GCP execution was not run because this test exercises the local fallback.

Checklist

Testing

• I have tested this change on a live environment and verified it works as intended.

Compliance & Style

• I have followed the Sample Guidelines.
• The README is updated with all relevant information (if applicable).
• If this PR adds a new sample directory: I updated the CODEOWNERS file.

---

Post-Approval Actions

• Please merge this PR for me once it is approved

[gemini-code-assist]

Code Review

This pull request adds an iter method to the SearchDocumentChunksResponse mock class in developer-knowledge/conftest.py to allow iterating over its results. There are no review comments, and I have no feedback to provide.