chore: mark chonk_browser.test.ts as flaky on Failed to fetch

[AztecBot]

Summary

Marks ivc-integration/src/chonk_browser.test.ts as flaky when the error matches "Failed to fetch" in .test_patterns.yml.

The CI failure on merge-train/fairies (http://ci.aztec-labs.com/1773654517173916) was caused by this browser WASM test timing out while fetching CRS/resources during the proving step — completely unrelated to #21472's nullifier witness changes.

CI log reference

• Test log: http://ci.aztec-labs.com/178008d500789810
• Error: Failed to fetch after ~67s during backend.prove() in headless Chrome
• Root cause: Network fetch timeout for CRS data or WASM chunks in browser environment

ClaudeBox log: https://claudebox.work/s/6526729432291118?run=1

[AztecBot]

Automatically closing this stale claudebox draft PR (no updates for 5+ days). Re-open if still needed.