Add hydration support for attachment content

[virtualLast]

Summary

Adds hydration support for Confluence attachment content.

The client already exposes attachment retrieval via:

$contentApi->children(
    $page,
    Content::CONTENT_TYPE_ATTACHMENT,
);

However, attachment results currently fail during hydration with:

HydrationException: Invalid content type: attachment

because AbstractContent::load() only supports page and comment.

Changes
Add ContentAttachment entity
Support Content::CONTENT_TYPE_ATTACHMENT in AbstractContent::load()
Add tests covering:

1. attachment hydration
2. attachment hydration within ContentSearchResult
3. Scope

This PR only adds read-only hydration support for attachment content.

It intentionally does not include:

1. attachment upload support
2. attachment download helpers
3. attachment metadata
4. API changes
5. hydrator refactoring

Fixes #18 

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

* **New Features**
  * Added support for loading Confluence attachments as content entities.
  * Attachments are now correctly recognized in content search results.

* **Tests**
  * Added coverage for loading attachments directly and through search results.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 7acfa991-6581-4f8d-a170-3e0896220620

📥 Commits

Reviewing files that changed from the base of the PR and between a43f758 and c42dd25.

📒 Files selected for processing (3)

• src/Entity/AbstractContent.php
• src/Entity/ContentAttachment.php
• tests/Entity/ContentTest.php

---

Walkthrough

Changes

Attachment hydration

Layer / File(s)	Summary
Attachment entity and loader mapping src/Entity/ContentAttachment.php, src/Entity/AbstractContent.php	Adds ContentAttachment and maps attachment content types in AbstractContent::load(), including the expanded return contract.
Direct and search-result validation tests/Entity/ContentTest.php	Adds shared attachment payload data and tests direct loading and loading through ContentSearchResult.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: adding attachment hydration support.
Linked Issues check	✅ Passed	The PR matches issue #18 by adding ContentAttachment, mapping attachment hydration, and covering direct and search-result loading.
Out of Scope Changes check	✅ Passed	The changes stay within attachment hydration and tests, with no unrelated functionality added.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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