fix(node:https,http2): accept Buffer-typed key/cert PEMs (#2132)

[proggeramlug]

Summary

Towards #2132. https.createServer({ key, cert }) and http2.createSecureServer rejected the Node-idiomatic Buffer form of key/cert (the shape fs.readFileSync('key.pem') returns when no encoding is supplied), printing "[node:https] no recognized PEM private key" and refusing to listen.

Why

The two helpers round-trip their options object through JSON.stringify → serde_json → .as_str(). A Buffer JSON-serializes as {"type":"Buffer","data":[...]}, so .as_str() returns None, the PEM buffer becomes empty, and the request bottoms out in the "no recognized PEM private key" branch. The node-core radar (#800) surfaced this as the diff-bucket cause for the three https tests cited in #2132:

• test-https-agent-abort-controller.js
• test-https-agent-keylog.js
• test-https-byteswritten.js

Fix

Add a shared json_value_to_pem_bytes helper in tls.rs that recognizes both the Node Buffer shape and a bare numeric array, and decodes them to bytes. Both option-parsers now thread byte buffers (not String) end-to-end — PEM is opaque to the TLS parsers, so there's no reason to require valid UTF-8.

Verified

• cargo test --release -p perry-ext-http-server --lib — 12 passed (4 new tests cover string PEM, Buffer-shape JSON, bare numeric array, empty/unknown fallbacks).
• cargo fmt --all -- --check clean.
• Direct re-run of the three diff tests against the local radar staging:
  • test-https-agent-abort-controller.js: was diff, now pass (both runtimes silent, exit 0).
  • test-https-agent-keylog.js: PEM step now succeeds; next blocker is rustls UnsupportedCertVersion on the Node test fixture (unrelated, distinct follow-up).
  • test-https-byteswritten.js: PEM step now succeeds; remaining diff is a missing console.log('ok') reachable code path (unrelated, distinct follow-up).

Scope

This is the first cluster from #2132 — it closes the PEM-Buffer source of diffs. Remaining buckets surfaced by the radar (server method dispatch, agent getName / addRequest, the UnsupportedCertVersion cluster) will be follow-up PRs.

Test plan

• cargo test --release -p perry-ext-http-server --lib
• cargo fmt --all -- --check
• Local node-core radar comparison on the three cited diff tests
• CI green