feat(protocol): add decision model request and response types

[nachiketb-nvidia]

What

Add public Decision Model types in crates/protocol:

• Boolean, Choice, and Score questions and answers.
• Structured input content, string IDs, optional ModelId, and shared Usage.
• Optional probability distributions and separate provider confidence.

Why

Provide the Decision Model protocol for future clients and algorithms.

Notes for reviewers

All fields are public, with derived serialization and deserialization. No constructors, validation helpers, or new error type. Score levels remain in the request; responses preserve provider scores and probabilities.

Validation: formatting, protocol compilation, and targeted Clippy passed. No tests added or run, as requested.

[github-actions]

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📥 Commits

Reviewing files that changed from the base of the PR and between fbabf51 and d6df493.

📒 Files selected for processing (2)

• crates/protocol/src/decision.rs
• crates/protocol/src/lib.rs

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Walkthrough

The protocol crate adds a public decision protocol. It validates request questions, response answers, numeric values, and probability distributions during construction and deserialization.

Changes

Decision Protocol

Layer / File(s)	Summary
Validated decision values crates/protocol/src/decision.rs	Adds checked probability, score, and provider-confidence types. Defines the distribution sum tolerance.
Decision request contract crates/protocol/src/decision.rs	Adds Boolean, choice, and score question types. Request construction and deserialization validate the question set, choice options, and score levels.
Request-bound response validation crates/protocol/src/decision.rs, crates/protocol/src/lib.rs	Adds response and answer types. Response validation checks question IDs, answer kinds, choices, scores, and optional distributions. Deserialization rejects duplicate map keys. The crate root exports the decision API.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to d6df4

This adds standalone decision protocol types with validation at construction and deserialization. No concrete merge-blocking risk was identified in the supplied material. The author reports that formatting, check and clippy passed. No tests were added.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 45.83% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 24 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding Decision Model request and response types to the protocol crate.

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_{I’m a rabbit with a protocol to try,
I check each answer as it hops by.
Choice IDs line up, scores stay in range,
Probabilities sum without a change.
I tuck the new decision types away,
Then nibble some clover to end the day.}

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