Replace httpbin with httpbingo in tests

[vishal-mb]

• I've read, understood, and done my best to follow the CONTRIBUTING guidelines.

This pull request includes (pick all that apply):

• Bugfixes
• New features
• Breaking changes
• Documentation updates
• Unit tests
• Other

Summary

Replace the unreliable public httpbin.org test dependency with httpbingo.org. Recent CI runs have received widespread HTTP 503 responses from httpbin.org, causing unrelated test failures.

Implementation

• Replaced all 52 httpbin.org test URLs with their httpbingo.org equivalents.
• Updated multipart response parsing from [String: String] to [String: [String]] because httpbingo represents form and file values as arrays.
• Kept the change test-only; production sources and CI configuration are unchanged.

Test Plan

• Ran the complete Swift package test suite locally: 219 tests, 0 failures.
• Verified concurrent downloads, images, multipart requests, delays, status codes, streaming, progress, and OAuth grant tests.
• Verified the GitHub Actions build passes.

Security Impact

• No security impact
• I have linked to a document that considers security impact or described the security impact below.