Return empty search result when providers return no jobs (remove mock example jobs)

[joelnishanth]

Motivation

• Users were being taken to https://example.com/... when clicking Apply because the search service generated mock jobs with example URLs when all providers returned no results.
• The intended behavior for a true no-results scenario is to show an empty/no-results UI state, not synthetic job cards with external links.

Description

• Remove the generateMockResults fallback and related mock job data from both apps/extension-chrome/src/shared/job-search-service.ts and apps/extension-firefox/src/shared/job-search-service.ts.
• Change searchJobs to return an explicit empty JobSearchResult (jobs: [], totalResults: 0, page: filters.page ?? 1, totalPages: 0) and cache that empty result when upstream providers yield no jobs.
• Preserve existing behavior for non-empty responses and keep caching/deduplication logic intact.

Testing

• Verified the code change by searching the codebase for the mock generator and example URLs with rg -n "generateMockResults|example.com/job", confirming removal of mock entries.
• Confirmed the new empty-result branch is present in both modified files by inspecting the updated source (apps/extension-chrome/src/shared/job-search-service.ts and apps/extension-firefox/src/shared/job-search-service.ts).
• Attempted to run the repository tests with npm --prefix apps/extension-chrome run -s test -- src/popup/popup.test.ts, but the test runner failed in this environment because vitest is not available, so unit tests were not executed here.

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Codex Task