Fix REST API docs request interceptor URL replacement

[tom2drum]

Description and Related Issue(s)

Fixes the request interceptor in the REST API documentation snippet to correctly replace default server URLs. The previous implementation chained .replace() calls which could cause incorrect URL replacements when both default server patterns were present in the URL (for example, https://megaeth.blockscout.com).

Proposed Changes

• Fixed the request interceptor logic in ui/apiDocs/utils.ts to check which default server URL is present before performing the replacement, ensuring only the correct server URL is replaced
• Updated MEGA network configuration from testnet to mainnet in configs/envs/.env.mega_eth and tools/preset-sync/index.ts

Breaking or Incompatible Changes

None.

Additional Information

The fix ensures that when the REST API documentation makes requests, it correctly replaces the default server URLs (blockscout.com/poa/core or eth.blockscout.com) with the configured API host, preventing potential URL replacement errors.

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request