feat: Add support for guardrails

[milesapnash]

Summary

Adds a new guardrails resource type to ADK, supporting both platform (toggle-only) and custom (prompt/action-defined) guardrails, synced through agent_settings/guardrails.yaml.

Motivation

Agent Studio now supports guardrails (platform-provided and custom) that were not yet representable in local ADK projects. This adds first-class support so they can be pulled, edited, and pushed like other resources.

Changes

• Added PlatformGuardrail and CustomGuardrail resource classes in src/poly/resources/guardrails.py, both stored in agent_settings/guardrails.yaml
• PlatformGuardrail supports only the enabled toggle against the platform's fixed guardrail catalog (GuardrailName proto enum), with YAML-friendly short names (e.g. jailbreak_defence)
• CustomGuardrail supports full create/update/delete with name, prompt, action, and enabled fields, plus reference validation against other resource types (global_functions, sms, handoff, attributes, variables, translations)
• Registered both resource classes via register_resource and exported them from src/poly/resources/__init__.py
• Added discover_resources/from_projection/to_yaml_dict/from_yaml_dict/validate implementations for both resource types
• Added guardrails fixture (agent_settings/guardrails.yaml) and updated test_project.json in the test fixture project
• Added extensive unit tests in src/poly/tests/resources_test.py

Test strategy

• Added/updated unit tests
• Manual CLI testing (poly <command>)
• Tested against a live Agent Studio project
• N/A (docs, config, or trivial change)

Checklist

• ruff check . and ruff format --check . pass
• pytest passes
• No breaking changes to the poly CLI interface (or migration path documented)
• Commit messages follow conventional commits

[linear-code]

AM-912

DEVP-178

[Ruari-Phipps]

Nice! LGTM!

[github-actions]

Coverage Report

Base (main)	PR	Change
75.9%	76.1%	+0.2% ✅