Normalized double slashes in digest auth URI to match request path

[Krishna2004V]

Fixes #6784

Summary

• Normalizes double slashes in digest auth URI to match the request path normalization done in HTTPAdapter.request_url().

Changes

• Added path normalization in build_digest_header() in src/requests/auth.py
• Added regression test test_DIGEST_AUTH_NORMALIZES_DOUBLE_SLASH_PATH

Problem

• When a URL contains double slashes (e.g., http://example.com//path), the actual HTTP request path gets normalized to /path, but the digest auth header was using //path. This mismatch caused authentication failures (400 Bad Request).

Solution

• Apply the same normalization to the path used in digest auth headers.