test(core): improve extractLinks error handling coverage

[saurabhsharma2u]

Improved error handling coverage for extractLinks in @crawlith/core.

Changes:

• Restored a try/catch block around cheerio.load in plugins/core/src/crawler/extract.ts to prevent the crawler from crashing on malformed HTML inputs that might cause cheerio to throw.
• Enhanced plugins/core/tests/extract.test.ts by:
  • Adding a test case should handle non-Error exceptions gracefully to ensure the catch block handles thrown strings/objects correctly.
  • Adding comments to clarify the cheerio mocking strategy using vi.mock and importOriginal.

Verified that all tests in extract.test.ts pass and coverage is maintained.

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PR created automatically by Jules for task 2828922681014962528 started by @saurabhsharma2u

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