Reject conflicting duplicate Content-Length headers

[Mirochill]

Summary

• validate all inbound content-length header fields before initializing the expected body length
• accept duplicate content-length headers only when all values agree
• add coverage for matching and conflicting duplicate content-length request headers

Closes #1316.

Validation

• git diff --check
• Not run: local h2 tests, tox, linters, or project tooling. This coordination workspace forbids local validation runs for third-party PRs; remote CI should provide the executable validation.

[Mirochill]

I missed that #1317 was already actively covering this issue. Closing this draft to avoid duplicating review effort; #1317 has the maintainer discussion and remote checks attached. Sorry for the noise.