Add CodSpeed performance benchmarks

[codspeed-hq]

This PR integrates CodSpeed for continuous performance tracking on httpx2.

What changed

Benchmark tests (tests/benchmarks/): 31 benchmarks covering the performance-critical paths of the library:

• URL parsing (7 benchmarks): Simple URLs, query/fragment handling, complex URLs with userinfo and port, IPv6, IDNA unicode hosts, long paths, percent-encoded characters.
• Headers (5 benchmarks): Construction from lists, key lookup, containment checks, iteration, and copying.
• Request/Response construction (7 benchmarks): GET and POST requests (JSON, form-encoded, with headers), text/JSON/HTML response construction.
• Content encoding (6 benchmarks): Small and medium byte payloads, string encoding, URL-encoded form data, small and nested JSON bodies.
• Decoders (4 benchmarks): Identity decoder, text decoder, line decoder, byte chunker.
• URL object (2 benchmarks): URL construction and copy_with.

CI workflow (.github/workflows/codspeed.yml): Runs benchmarks in simulation mode on every push to main and on pull requests, using OIDC authentication. Follows the existing pinning strategy (commit hashes with version comments).

Dependencies: Added pytest-codspeed to the dev dependency group.

README: Added CodSpeed badge.

Next steps

• Once this PR is merged and the workflow runs on main, CodSpeed will start tracking performance and reporting regressions on future pull requests.
• To run benchmarks locally: uv run codspeed run -m simulation -- pytest tests/benchmarks/ --codspeed
• Additional benchmarks can be added to tests/benchmarks/ following the same benchmark fixture pattern.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 31 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_response_json: 141.8 µs
• test_line_decoder: 49.6 µs
• test_text_decoder: 43.6 µs
• test_headers_contains: 23.5 µs
• test_encode_content_bytes_medium: 20.3 µs
• test_encode_content_bytes_small: 30.5 µs
• test_request_post_form: 203.1 µs
• test_encode_content_str: 23.8 µs
• test_request_post_json: 202.5 µs
• test_encode_json_small: 56.5 µs
• test_encode_urlencoded_data: 78.5 µs
• test_byte_chunker: 89.5 µs
• test_identity_decoder: 10.1 µs
• test_response_html: 119.1 µs
• test_response_text: 131.1 µs
• test_urlparse_with_query_and_fragment: 124.8 µs
• test_url_construction: 135.2 µs
• test_url_copy_with: 162.9 µs
• test_urlparse_complex: 142.2 µs
• test_urlparse_ipv6: 122.1 µs
• ...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

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