feat(python): expose decision call steps

[nachiketb-nvidia]

Python hosts currently raise NotImplementedError when an algorithm requests a decision. This exposes Step.CallDecision so a host can read the request and target, then return a response or client error.

Closes SWITCH-1695.

How

• Add DecisionCall with algorithm, request, model, respond, and fail. Requests and responses use the existing dictionary conversion helpers and the neutral Decision Model protocol.
• Preserve single completion, provider-error fallback, and Rust drop/cancellation behavior. Invalid response dictionaries leave the call available for a corrected response.
• Add TaskClassifierConfig.decision(...) to select the existing relative-advantage judge from Python, using Rust's default routing and fallback settings.
• Update public exports and the run_stream return type.

Independent of #907; this uses the protocol, driver, and classifier already on main.

Validation

• Built the Python extension; all 20 existing tests in tests/test_libsy_minimal_bindings.py passed.
• Manual Python checks passed for both routing outcomes, provider failure, malformed-response recovery, duplicate replies, dropped calls, and cancellation.
• Focused Clippy, Ruff, mypy, formatting, and diff checks passed.
• No test files added or changed. No live provider calls.

Summary by CodeRabbit

• New Features
  • Added decision-based classification configuration, with support for setting a cutoff, candidate models, required evidence, and optional instructions.
  • Streaming workflows can now receive decision calls and respond to or fail them using Python.
  • Decision calls expose their algorithm, request, and selected model.

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository: NVIDIA-NeMo/Switchyard/.coderabbit.yaml
• Review profile: CHILL
• Plan: Enterprise
• Run ID: 0d373063-cd52-40dd-868a-5f5a862697a2

📥 Commits

Reviewing files that changed from the base of the PR and between 1aebea4 and 14ed0b6.

📒 Files selected for processing (3)

• crates/switchyard-py/src/libsy_bindings.rs
• switchyard/libsy/__init__.py
• switchyard_rust/libsy.py

Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 11 remain after this review.

---

Walkthrough

The Python API now supports decision-classifier configuration and exposes Rust decision calls as Python stream steps. Python code can inspect each call and submit a response or failure.

Changes

Python decision-call support

Layer / File(s)	Summary
Decision configuration and API crates/switchyard-py/src/libsy_bindings.rs, switchyard_rust/libsy.py	TaskClassifierConfig.decision constructs a decision judge from a cutoff, candidate models, evidence, and optional instructions. The Python declarations add DecisionCall methods and include Step.CallDecision in the stream types.
Runtime bridge and package exports crates/switchyard-py/src/libsy_bindings.rs, switchyard_rust/libsy.py, switchyard/libsy/__init__.py	The bindings convert Rust decision calls into Python DecisionCall objects. The package exports the class, and respond and fail forward completions to the waiting Rust call.

Priority: ➖ Normal

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

Merge Risk: ⚪ Minimal · up to 14ed0

The decision-call API is mergeable after normal checks; no actionable issue is established by the supplied evidence.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 38.10% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: exposing decision-call steps to Python hosts.
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.

• Fix all pre-merge checks with AI

• Autopilot · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

_{A rabbit checks the stream at dawn,
A decision call hops safely on.
It sends a response, or reports a fail,
Then shares its model along the trail.
The Python burrow welcomes all.}

_{Comment @coderabbitai help to get the list of available commands.}

[ayushag-nv]

lgtm