Add python-multipart Content-Length fix references

[cookesan]

Adds python-multipart Content-Length fix references to GHSA-v9pg-7xvm-68hf.

Audit refresh, July 8, 2026:

• Confirmed the live PR is open, ready for review, merge-clean, and the visible advisory check is green.
• Rechecked the merge-base diff: exactly one advisory JSON file changes (advisories/github-reviewed/2026/06/GHSA-v9pg-7xvm-68hf/GHSA-v9pg-7xvm-68hf.json), JSON parses, git diff --check passes, and the exact-head worktree is clean.
• Revalidated the added public references and package-version evidence in the diff; all resolve through their source site or registry.

[cookesan]

Review note: this expands GHSA-v9pg-7xvm-68hf with the python-multipart Content-Length fix commit plus 0.0.31 release and PyPI package evidence. I checked the PR head JSON against the OSV schema, confirmed commit c814948acf509cef7881fa75c969969b19239bbf resolves to the negative Content-Length rejection fix, confirmed the release URL resolves, and confirmed PyPI has python-multipart==0.0.31; the affected PyPI range, CVSS score, alias, and package metadata stay unchanged.