feat(sglang): define sidecar executable contract

[connorcarpenter15]

Summary

• Make dynamo-sglang-sidecar parse its process arguments explicitly before constructing SglangSidecarEngine.
• Add SglangSidecarEngine::from_parsed_args so the executable has a clear parsed-CLI boundary.
• Add unit and executable-level coverage for the SGLang-managed --sglang-endpoint contract.
• Document the expected SGLang supervision, failure-propagation, and graceful-shutdown behavior.
• Ensure changes under lib/sglang-sidecar trigger SGLang CI.

Motivation

SGLang should be able to supervise Dynamo as a real child executable rather than importing an in-process Python module. The executable remains a thin composition of the native SGLang adapter and dynamo_backend_common::run, while SGLang owns process startup and shutdown.

python3 -m sglang.launch_server \
    <args> \
    --grpc-port 30001 \
    --sidecar-executable dynamo-sglang-sidecar

Packaging or bundling the executable is intentionally deferred.

Scope

This extracts the Rust executable portion of #11636. It does not include the Python entry-point changes or move lib/sglang-sidecar into lib/backend; those are separate changes.

Validation

• cargo fmt --all -- --check
• cargo metadata --no-deps --format-version 1
• git diff --check

The focused local Cargo test progressed through the sidecar crate and its dependencies but could not finish because the workspace filesystem ran out of space; draft PR CI will provide the full build and test result.

Summary by CodeRabbit

• New Features

  • The SGLang sidecar now supports launching with the --sglang-endpoint command-line option.
  • Improved command-line help describes endpoint configuration and SGLang-managed operation.

• Documentation

  • Added usage instructions covering build, startup, supervision, timeouts, and endpoint discovery.
  • Clarified configuration for prefilling deployments.

• Tests

  • Added coverage verifying endpoint argument parsing and executable help output.

• Chores

  • Updated CI routing so sidecar changes trigger the appropriate checks.

[github-actions]

🌿 Fern Docs Preview: https://nvidia-preview-019f6cc7-3048-772b-bdd3-84e560d32986.docs.buildwithfern.com/dynamo/dev

[datadog-official]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 41.10% (-7.57%)

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🔗 Commit SHA: 386dc5c | Docs | Give us feedback!}

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[coderabbitai]

Walkthrough

The SGLang sidecar now parses its endpoint from CLI arguments, constructs the engine from parsed arguments, documents launcher supervision requirements, exposes updated help text, validates the executable contract, and routes sidecar changes to SGLang CI jobs.

Changes

SGLang sidecar CLI integration

Layer / File(s)	Summary
CLI contract and sidecar documentation lib/sglang-sidecar/README.md, lib/sglang-sidecar/src/args.rs, lib/sglang-sidecar/Cargo.toml	The sidecar’s description, endpoint parsing test, CLI wording, build instructions, and SGLang-managed launcher contract are updated.
Parsed-argument engine bootstrap lib/sglang-sidecar/src/engine.rs, lib/sglang-sidecar/src/main.rs	The engine adds from_parsed_args, while the binary parses Args and passes them into engine construction.
Executable validation and CI routing lib/sglang-sidecar/tests/executable.rs, .github/filters.yaml	Executable help output is checked for the endpoint contract, and sidecar changes are included in SGLang CI filtering.

Estimated code review effort: 2 (Simple) | ~15 minutes

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The PR description covers the change, but it does not follow the required template and omits the required Related Issues and reviewer-start sections.	Rewrite it with the repository template: Overview, Details, Where should reviewer start?, and Related Issues with a linked issue or the no-issue checkbox.
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main change: defining the SGLang sidecar executable contract.
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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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/sglang-sidecar/src/main.rs`:
- Around line 6-9: In the module documentation comment, rephrase the sentence
around from_parsed_args so it clearly states that it builds the WorkerConfig
that run needs synchronously, preserving the surrounding bootstrap and
runtime-harness description.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

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Run ID: fa9183e0-95a8-4771-97c4-462f578755f0

📥 Commits

Reviewing files that changed from the base of the PR and between 01d2cb2 and 386dc5c.

📒 Files selected for processing (7)

• .github/filters.yaml
• lib/sglang-sidecar/Cargo.toml
• lib/sglang-sidecar/README.md
• lib/sglang-sidecar/src/args.rs
• lib/sglang-sidecar/src/engine.rs
• lib/sglang-sidecar/src/main.rs
• lib/sglang-sidecar/tests/executable.rs