feat: add Agent Studio test case resource support

[Ruari-Phipps]

Summary

Adds local ADK support for Agent Studio test cases, including YAML resources under test_suite/, protobuf sync commands, pull/push integration, documentation, and unit tests.

Motivation

Agent Studio now supports simulated conversation tests with prompt and function-call assertions. The ADK needs to read, write, validate, and sync these resources locally so projects can manage tests alongside other agent configuration.

Changes

• Add TestCase, TestCaseAssertion, TestCaseTags, and function-call assertion types in src/poly/resources/test_suite.py
• Register test cases in project resource mapping and sync client projection parsing
• Update protobuf bindings (including new testing_pb2) and command types for create/update/delete/assertions/tags
• Add tests.md documentation and link it from the main docs index
• Add unit tests in resources_test.py and project discovery coverage in project_test.py
• Add sample test cases to the test project (test_suite/greeting_flow_test.yaml, test_suite/webchat_smoke_test.yaml)
• Set __test__ = False on resource classes to avoid pytest collection warnings
• Validate test case language against configured project languages (default + additional)
• Validate function call assertion names against known global functions

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

Screenshots / Logs

name: Test Weather flow
scenario: Ask for the weather for berlin.
channel: voice
language: es-ES
tags:
- weather
prompt_assertions:
- It gives the weather
- It gives the weather for Berlin
function_call_assertions:
- name: get_weather
  arguments:
  - parameter_name: city
    expected_value: Berlin
    value_type: string

[github-actions]

Coverage Report

Base (main)	PR	Change
69.6%	69.9%	+0.3% ✅

Changed file coverage

File	Coverage	Change
poly/handlers/sync_client.py	24.5%	-0.7% ⚠️