Fix: Merge URL query parameters instead of replacing them

[veeceey]

Summary

Fixes #3621

When a URL already contains query parameters and additional params are passed, httpx was dropping the original URL parameters instead of merging them. This was inconsistent with the requests library behavior.

Problem

url = 'https://api.com/get?page=1&sort=desc'
params = {'size': 10, 'filter': 'active'}

# Before this fix
response = httpx.get(url=url, params=params)
# URL sent: https://api.com/get?size=10&filter=active
# (page=1 and sort=desc were lost!)

# After this fix
response = httpx.get(url=url, params=params)
# URL sent: https://api.com/get?page=1&sort=desc&size=10&filter=active
# (all parameters are present)

Solution

Modified URL.__init__() in _urls.py to:

1. Extract existing query parameters from the URL
2. Merge them with new parameters from the params argument
3. When the same key exists in both, new params override old ones (merge semantics)

Changes

• Modified httpx/_urls.py: Updated param handling to merge instead of replace
• Added comprehensive tests in tests/models/test_url.py

Test Cases

Merge existing and new params:

url = httpx.URL("https://example.com/get?page=post&s=list", params={"pid": 0, "tags": "test"})
# Result: page=post&s=list&pid=0&tags=test

Override with same key:

url = httpx.URL("https://example.com/get?a=old", params={"a": "new", "b": "2"})
# Result: a=new&b=2 (a was overridden)

Empty params preserves existing:

url = httpx.URL("https://example.com/get?x=5", params={})
# Result: x=5 (preserved)

No existing params works normally:

url = httpx.URL("https://example.com/get", params={"a": "1"})
# Result: a=1

Testing

• ✅ Manual testing with various URL/param combinations
• ✅ Added regression tests
• ✅ Verified backward compatibility with existing tests

[veeceey]

All CI passing on Python 3.9-3.13, ready for merge