feat(tokenspeed): propagate stochastic sampling seeds

[lucifer1004]

Motivation

The TokenSpeed gRPC scheduler had no way to receive a per-request sampling seed: the OpenAI-style seed parameter was dropped at the router, so stochastic sampling could not be made reproducible, and fan-out (n > 1) choices all shared whatever seed the engine happened to pick.

What this changes

• tokenspeed_scheduler.proto: SamplingParams gains optional uint64 sampling_seed = 23; omission delegates seed selection to the engine (proto field is new — upstream does not have it).
• translate_sampling_tokenspeed maps the request seed through.
• Fan-out derives distinct per-choice seeds (sampling_seed = seed + i for choice i) inside fan_out_n, so n > 1 requests are reproducible and choices differ from each other. No explicit seed → None preserved for every choice.

Tests

8/8 zmq_client::tests::tokenspeed including tokenspeed_fan_out_derives_explicit_sampling_seeds (explicit seed → 7/8/9 derived across choices; omitted → None throughout); smg-grpc-client 72/0; fmt/clippy clean on touched crates.

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📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added optional per-request sampling seed support for chat, completion, and text generation.
  • Explicit seeds now produce deterministic results across requests.
  • Multiple generated outputs receive distinct, deterministic seed values.

• Bug Fixes

  • Invalid negative sampling seeds are rejected with a clear error.
  • Sampling seeds are now consistently preserved when requests are forwarded.

Walkthrough

The change adds an optional per-request sampling seed. Request conversion validates signed seeds, gateway translation forwards them, and TokenSpeed fan-out derives indexed seeds for multiple samples.

Changes

Sampling seed support

Layer / File(s)	Summary
Request seed contract and conversion crates/grpc_client/proto/tokenspeed_scheduler.proto, crates/grpc_client/src/tokenspeed_scheduler.rs	SamplingParams accepts an optional sampling_seed. Chat and completion requests reject negative seeds before conversion to u64. Plain generation requests copy the optional seed.
TokenSpeed translation and fan-out model_gateway/src/routers/grpc/zmq_client.rs	TokenSpeed wire parameters receive the request seed. Fan-out adds each sample index with wrapping arithmetic. Tests cover direct mapping, indexed seeds, and unset seeds.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to 13587

Chat and completion requests can still lose an explicitly provided sampling seed, causing reproducibility to fail for those request types. The PR is not merge-ready until those paths forward the seed with defined precedence.

Suggested reviewers: slin1237, catherinesue

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant SamplingParams
  participant tokenspeed_scheduler
  participant zmq_client
  participant TokenSpeed
  Client->>SamplingParams: Set optional sampling_seed
  SamplingParams->>tokenspeed_scheduler: Convert request parameters
  tokenspeed_scheduler->>zmq_client: Pass validated or copied seed
  zmq_client->>TokenSpeed: Fan out requests with indexed seeds
  TokenSpeed-->>zmq_client: Process seeded or engine-seeded requests

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: propagating stochastic sampling seeds for TokenSpeed.
Description check	✅ Passed	The description explains seed propagation, fan-out behavior, omitted seeds, and related tests.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[lucifer1004]

Note for reviewers: this PR and #2226 (canonical engine-load metrics) both touch crates/grpc_client/src/tokenspeed_scheduler.rs (and zmq_client.rs). Disjoint hunks; the second to land may need a trivial rebase.

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Actionable comments posted: 1

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Inline comments:
In `@crates/grpc_client/src/tokenspeed_scheduler.rs`:
- Around line 650-666: Update chat_sampling_seed and completion_sampling_seed to
return the request’s sampling_seed directly when present, falling back to
unsigned_sampling_seed for the legacy seed field only when sampling_seed is
absent. Define and preserve this precedence for both request types, and add
conversion tests covering explicit sampling_seed, legacy seed, and both fields
set.

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ℹ️ Review info

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Run ID: 910c65bd-dabd-4ffa-8d78-107ae8cd52c1

📥 Commits

Reviewing files that changed from the base of the PR and between c1f671e and 1358731.

📒 Files selected for processing (3)

• crates/grpc_client/proto/tokenspeed_scheduler.proto
• crates/grpc_client/src/tokenspeed_scheduler.rs
• model_gateway/src/routers/grpc/zmq_client.rs

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