fix: honor multipart part limits at the boundary

[hansu650]

Summary

ParseMultipartConfig.max_parts is documented as the maximum number of accepted parts, but the opening delimiter creates an empty first element in bytes.split(). Counting that element rejects a normal request containing exactly the configured limit (for example, one part with max_parts=1, or 100 parts with the default configuration).

Remove only that leading empty element before checking the limit. The bounded maxsplit and the pre-parse limit check are unchanged. This also permits an empty body containing just the closing boundary when max_parts=0.

Regression coverage

Four new test methods cover:

• Limits 0, 1, 2, and the default 100, including below/at/above-limit cases.
• Repeated field names and mixed file/text parts, so the limit counts parts rather than distinct names.
• Rejection before either output dictionary is populated.
• Preservation of the limit for a body without an initial delimiter and for repeated empty segments.

Before the production change, the multipart test class reports five subtest errors: the four exact-limit cases and the two-part file/text case. The entire httputil_test module passes after the fix.

Validation

Tested in a dedicated Python 3.13.15 environment on Windows, using the repository's pinned lint/docs dependencies.

• python -bb -m tornado.test tornado.test.httputil_test: 62 tests passed.
• python -bb -m tornado.test tornado.test.httpserver_test tornado.test.http1connection_test: 69 passed, 6 platform/configuration skips.
• black --check --diff tornado demos and flake8: passed. Task-local environment/cache directories were excluded from flake8.
• All three configured mypy checks passed: Linux and Windows targeting Python 3.11, and Linux targeting Python 3.13.
• Sphinx coverage and doctest builds with -q -E -n -W: passed; 77 doctests, no failures.

Local platform limitations

The complete python -bb -m tornado.test --fail-if-logs=false run reports 1,249 tests, 112 skips, and 5 IPv6 connection errors. The same five errors occur on an unmodified checkout of 0096f2897c98facdcd9716009ee934a7381af5ef (1,245 tests, 112 skips). A standalone standard-library socket check also fails to connect to ::1 with WinError 10013 while IPv4 loopback succeeds. No tests were newly skipped or changed to hide these errors. The log-handling flag is the repository's Windows CI setting.

Strict Sphinx HTML builds of both the patch and that unmodified baseline report the same warning at docs/releases/v6.5.0.rst:75: unresolved bind_unix_socket on Windows. This unrelated documentation was not changed.

The C extension was not built locally, and the complete tox -e lint,docs,py3 command is not claimed as passing. Cross-platform checks remain for CI.

AI assistance: this patch and its tests were prepared with Codex.