Add policy languages and runtimes doc

[renuka-fernando]

Purpose

$subject

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Summary

This PR adds comprehensive documentation for policy authoring and runtime execution on the API Gateway, introducing a new guide that covers Go and Python policy development.

Changes

Documentation additions:

• New guide: docs/gateway/policy-languages-and-runtimes.md documenting how the Gateway Runtime executes policies in both Go and Python
• Updated docs/gateway/README.md to link to the new guide

Content Overview

The new documentation covers:

• Runtime Architecture: How the Gateway Runtime executes policies, with shared fundamentals across both languages
• Policy Anatomy: Unified YAML schema for policy definitions (name, version, parameters, systemParameters) used by both languages
• Implementation Details:
  • Go policies compiled directly into the Policy Engine via ext_proc protocol (zero IPC/serialization overhead)
  • Python policies executed by a separate Python Executor communicating over local gRPC via Unix Domain Socket
• Project Structure: Representative layouts for both Go (including policy-definition.yaml, go.mod, policy.go) and Python (including pyproject.toml, requirements.txt, src/.../policy.py)
• Build Integration: Integration fields in build.yaml for each language (gomodule for Go, pipPackage for Python)
• Language Selection Guidance: Decision matrix and recommendations for choosing between Go and Python, with Go as the default and Python as beta

Impact

Provides clear guidance for developers on dual-language policy authoring, helping them understand runtime architecture, project structure, build processes, and language-specific considerations.

Walkthrough

This pull request adds documentation for policy language support in the Gateway Runtime. It introduces a new page explaining how the Gateway Runtime executes policies written in Go and Python. The documentation covers the shared YAML schema for policy definitions, describes the distinct execution models for each language (Go compiled directly into the Policy Engine versus Python executed via a separate Python Executor over gRPC), provides representative project layouts for both languages, and includes build configuration integration details and a decision matrix comparing language trade-offs.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is largely incomplete, providing only the Purpose section placeholder without required details from the template.	Complete the PR description by filling in all required sections: Goals, Approach, User stories, Documentation, Automation tests, Security checks, Samples, Related PRs, and Test environment.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding documentation about policy languages and runtimes.
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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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch python-policy-engine

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