Merge params with existing URL query parameters instead of replacing

[veeceey]

Summary

• Fixes When the URL contains request parameters and the params parameter is set, the request parameters in the URL will disappear unexpectedly. #3621: When a URL already contains query parameters (e.g. https://example.com/path?page=1&s=list) and the params argument is also provided, the existing query parameters were silently dropped and replaced by only the new params. This was inconsistent with the requests library and surprised users.
• Changed Request.__init__ to use URL(url).copy_merge_params(params) instead of URL(url, params=params), so existing URL query parameters are preserved and merged with the new ones.
• Updated test_request_params to reflect the new merge behavior and added an additional test case for overlapping keys.

Before (broken)

request = httpx.Request("GET", "https://example.com?page=1&s=list", params={"pid": 0})
print(request.url)
# https://example.com?pid=0   <-- page and s are lost!

After (fixed)

request = httpx.Request("GET", "https://example.com?page=1&s=list", params={"pid": 0})
print(request.url)
# https://example.com?page=1&s=list&pid=0   <-- all params preserved

Test plan

• Existing test_request_params updated and passes
• New test case for overlapping parameter keys added
• All 173 model and client tests pass
• Verified the exact reproduction case from the issue works correctly

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