Match exact Content-Length headers

[OskarEichler]

Summary

Require Content-Length to appear as a complete header line instead of accepting the token inside another header name.

Reproduction

A frame containing X-Content-Length is currently accepted as if it supplied Content-Length. The focused external model fails on 3.17.0.6 and current main, then passes with this change by raising ArgumentError for a missing or invalid header.

Verification

• Existing suite: 6 runs, 11 assertions, zero failures
• Focused malformed-header model passes
• All Ruby syntax checks pass
• Steep passes
• Generated source remains clean
• Rebuilt gem preserves expected package contents
• RuboCop LSP initialize/shutdown integration passes
• Rails 8.1.3.1 loads the candidate

No tests were changed. This source-only patch was identified and verified during an AI-assisted dependency audit.