chore(bigquery-firestore-export): fix broken links

[HassanBahati]

Description

Fixes broken GitHub URLs in the bigquery-firestore-export extension. The sourceUrl and releaseNotesUrl in extension.yaml were missing the tree/blob path segments, so they pointed to invalid GitHub links.

Changes Made

• sourceUrl: Added tree so the link uses the correct GitHub directory URL format:
  .../firebase-extensions/tree/main/bigquery-firestore-export (was .../firebase-extensions/main/bigquery-firestore-export).
• releaseNotesUrl: Switched to the correct file URL format using blob:
  .../firebase-extensions/blob/main/bigquery-firestore-export/CHANGELOG.md (was .../firebase-extensions/main/bigquery-firestore-export/CHANGELOG.md).

Testing

• Verified the corrected URLs in a browser.

Tests Run (if applicable)

• Unit tests — N/A
• Integration tests — N/A
• Manual testing — URL format check and optional browser verification

Additional Notes

• No behavior change; only metadata links in extension.yaml were updated.

[cabljac]

lgtm, good spot! can we update CHANGELOG