Preserve existing URL query parameters when params are provided

[ferstar]

Summary

• preserve query parameters already present in a URL when additional params are provided
• append same-name parameters instead of applying dictionary-style replacement
• keep copy_with(params=...) and the query manipulation helpers replacement semantics unchanged
• cover URL, Request, empty parameter inputs, same-name values, explicit queries, and invalid URL types

Closes #1173

Testing

• uv run --frozen coverage run -m pytest -q — 1998 passed, 1 skipped
• uv run --frozen coverage report --fail-under=100 — 100%
• uv run --frozen ruff format --check --diff src/httpx2 tests/httpx2 src/httpcore2 tests/httpcore2 benchmark
• uv run --frozen ruff check
• uv run --frozen mypy src/httpx2 tests/httpx2 src/httpcore2 tests/httpcore2 benchmark
• uv run --frozen python scripts/unasync.py --check

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This does not apply to typos.)
• I have added a test for each introduced behavior and kept the change atomic.
• Documentation changes are not required for this bug fix.

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing ferstar:codex/fix-preserve-url-query-params (063c764) with main (9fdafff)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩