perf: transcode Python request plane directly (cherry-pick of #11104)

[nv-yna]

Summary

Cherry-pick of #11104 ("perf: transcode Python request plane directly") onto feat/deepseek_v4_aa.

Mirrors the methodology from #11138 (cherry-pick of #10437 onto this same branch).

Changes included (verbatim from #11104)

• lib/bindings/python/rust/python_payload.rs — new Python-native msgpack payload encoder
• lib/bindings/python/rust/engine.rs — use Python payload codec directly (avoid JSON round-trip)
• lib/bindings/python/rust/lib.rs — wiring
• lib/runtime/src/pipeline/network.rs — RequestPlanePayloadCodec made pub; new default is Msgpack
• lib/runtime/src/pipeline/network/ingress/push_handler.rs — serialization error handling
• lib/runtime/src/metrics/prometheus_names.rs — SERIALIZATION error variant added
• New tests: test_request_plane_python_payload.py, bidirectional_e2e_json.rs
• tests/frontend/realtime_echo_worker.py — updated for codec changes

Divergence / conflict resolution

feat/deepseek_v4_aa is ~236 commits behind main. The cherry-pick applied cleanly with no conflicts — git auto-resolved all 5 touched files (Cargo.lock, Cargo.toml, lib.rs, prometheus_names.rs, network.rs) because #10437 (the prerequisite codec PR) was already on feat as of #11138.

No ambient-main symbols were introduced. Diff counts exactly match the source commit: 13 files, +1349/-270.

Build result

✅ PASS — both mandatory checks clean:

• cargo check -p dynamo-runtime --tests → Finished dev profile in 23.77s
• cargo check (Python bindings crate) → Finished dev profile in 54.40s

Do NOT merge

Leave merging to @yna — rebase-merge preferred (same as #11138).

Automated cherry-pick triggered by merge of #11104.

[datadog-official]

⚠️ Warnings

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