internal/transport: avoid mutating request URL for https+insecure

[cuishuang]

transport.RoundTrip shallow-copies the request when handling https+insecure, but the copied request still shares the original *url.URL.

As a result, changing the copied URL scheme from https+insecure to https also mutates the caller's request.

Copy the url.URL value and update the copied request to point to the new value. Add a regression test that verifies the HTTPS request succeeds and the caller's URL scheme remains unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.20%. Comparing base (ef3492d) to head (08dd1b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1017      +/-   ##
==========================================
- Coverage   67.35%   67.20%   -0.15%     
==========================================
  Files          44       45       +1     
  Lines        7793     7846      +53     
==========================================
+ Hits         5249     5273      +24     
- Misses       2112     2133      +21     
- Partials      432      440       +8     

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