refactor: add null guards in db mappers and fix JsonResponse return type for Psalm 6

[ChristophWurst]

Summary

• LocalMessageMapper: add null guard for getLocalMessageId() before using as array key in getAllForUser, findDue, findDueDrafts; use ?? [] for getRecipients() in updateWithRecipients
• MessageMapper: extract getMessageId() to a local variable with null guard before using as array key in updateTags and findRelatedData
• JsonResponse: align @return static → @return self on errorFromThrowable to match the actual PHP return type hint

These are Psalm 6 PossiblyNullArrayOffset and MoreSpecificReturnType errors that remain after the Psalm v6 upgrade in #11224.

Test plan

• PHP unit tests pass (composer test:unit)
• Psalm static analysis passes (composer psalm)

Part of #11224

[ChristophWurst]

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📥 Commits

Reviewing files that changed from the base of the PR and between cf0f574 and 19af1af.

📒 Files selected for processing (3)

• lib/Db/LocalMessageMapper.php
• lib/Db/MessageMapper.php
• lib/Http/JsonResponse.php

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📝 Walkthrough

Walkthrough

This PR addresses null-safety in data mapper operations by preventing null keys from being used in array lookups across three database mapper classes. In LocalMessageMapper, three query methods now skip recipient and attachment rows where localMessageId is null before building their maps, and updateWithRecipients() normalizes nullable recipients to an empty array. In MessageMapper, both updateTags() and findRelatedData() now check for non-null message_id before accessing the tags map. Additionally, a PHPDoc annotation in JsonResponse::errorFromThrowable() is updated from static to self.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: adding null guards in db mappers and fixing JsonResponse return type annotation for Psalm 6 compatibility.
Description check	✅ Passed	The description comprehensively covers all changes across the three modified files, explains the purpose (addressing Psalm 6 errors), and confirms testing has passed.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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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