docs: define Rust Python harness structure

[yujonglee-berri]

TLDR

Problem this solves:

• Migration harness organization and runner responsibilities need agreement

How it solves it:

• Document the expected structure and local execution rules
• Keep native Rust tests beside their implementation

User Flow

Before: contributors lack a documented migration testing layout

1. They consult the harness contributor instructions
2. They find no guidance on organizing migration checks

After: contributors can follow an agreed migration testing layout

1. They consult the harness contributor instructions
2. They find strategy responsibilities and independent execution guidance

Relevant issues

Linear ticket

Pre-Submission checklist

• I have added meaningful tests (documentation only)
• Relevant tests pass locally (not run, documentation only)
• My PR passes all required CI/CD checks
• My PR only solves one specific problem
• Greptile Confidence Score is at least 4/5

Screenshots / Proof of Fix

Before (993766b)

1. Run git ls-tree 993766be0e -- tests/rust-python-harness/AGENTS.md
2. Observe no entry

After (fced0d5)

1. Run git show fced0d5bb3:tests/rust-python-harness/AGENTS.md
2. Observe the expected directory tree and concise responsibility bullets
3. Run git diff --check 993766be0e...fced0d5bb3 and observe no whitespace errors

Type

Documentation

Caveats (if any)

Low

• Documents the intended structure; implementation remains separate

Final Attestation

• Reviewed the documentation against the agreed harness design

[devin-ai-integration]

PR #39456 (1 new docs file, +44/-0, no sensitive areas) labeled risk:low; it is the only risk label. No enterprise label, so Linear routing was skipped.

[greptile-apps]

Greptile Summary

Adds contributor guidance defining the intended organization and responsibilities of the Rust/Python migration harness.

• Documents the expected strategy, shared-utility, and entry-point layout.
• Defines independent execution, parity checking, mapping validation, and native Rust test placement.
• Establishes that the harness runs locally without CI integration.

Confidence Score: 5/5

The documentation-only change appears safe to merge.

The new file records an explicitly forward-looking harness design and does not alter runtime, build, test, or security behavior.

Important Files Changed

Filename	Overview
tests/rust-python-harness/AGENTS.md	Adds concise, forward-looking harness organization and runner-responsibility guidance without changing executable behavior.

_{Reviews (1): Last reviewed commit: "doc" | Re-trigger Greptile}