feat: add support for templates API

[jayanratna]

This PR adds support for the templates API.

---

Summary by cubic

Add full support for the Templates API, including CRUD, publish, and duplicate operations. Also adds stricter ID validation for single-resource requests.

• New Features

  • New templates service: get, create, list, update, remove, publish, duplicate.
  • New Template resource with typed properties.
  • Client and ServiceFactory wired to expose templates; Payload helpers for publish/duplicate send an empty JSON body ({}).

• Bug Fixes

  • Payload::get now throws an InvalidArgumentException when the ID is an empty string.

^{Written for commit 04a3612. Summary will update automatically on new commits.}

[cubic-dev-ai]

3 issues found across 11 files

Prompt for AI agents (all 3 issues)

Understand the root cause of the following 3 issues and fix them.


<file name="src/Template.php">

<violation number="1" location="src/Template.php:13">
`$published_at` can be null for draft templates, so this docblock should allow null to keep type information accurate for consumers.</violation>
</file>

<file name="tests/Fixtures/Template.php">

<violation number="1" location="tests/Fixtures/Template.php:21">
The first list entry uses the variable ID instead of the template ID, so the fixture data is inconsistent with template() and can break tests or clients relying on matching IDs.</violation>
</file>

<file name="tests/Transporters/HttpTransporter.php">

<violation number="1" location="tests/Transporters/HttpTransporter.php:215">
The UnserializableResponse thrown for an empty JSON body uses the message &#39;Empty response body&#39;, but this assertion expects &#39;Empty&#39;, so the test will always fail. Update the expected message to match the actual exception text.</violation>
</file>

_{React with 👍 or 👎 to teach cubic. Mention @cubic-dev-ai to give feedback, ask questions, or re-run the review.}

[cubic-dev-ai]

The first list entry uses the variable ID instead of the template ID, so the fixture data is inconsistent with template() and can break tests or clients relying on matching IDs.

Prompt for AI agents

Address the following comment on tests/Fixtures/Template.php at line 21:

<comment>The first list entry uses the variable ID instead of the template ID, so the fixture data is inconsistent with template() and can break tests or clients relying on matching IDs.</comment>

<file context>
@@ -0,0 +1,58 @@
+        &#39;text&#39; =&gt; &#39;Hello, world!&#39;,
+        &#39;variables&#39; =&gt; [
+            [
+                &#39;id&#39; =&gt; &#39;e169aa45-1ecf-4183-9955-b1499d5701d3&#39;,
+                &#39;key&#39; =&gt; &#39;user_name&#39;,
+                &#39;type&#39; =&gt; &#39;string&#39;,
</file context>