[codex] Update Firebase 12.6.0 documentation

[AdamEssenmacher]

Summary

• update the README Firebase package status for the published 12.6.0 release line
• move AppDistribution and InAppMessaging into the published Firebase package set
• normalize Firebase NuGet documentation examples to the actual published 12.6.0 versions
• update the AppCheck NuGet sample references to 12.6.0

Validation

• rg "12\\.5\\.0\\.4|12\\.6\\.0\\.[0-9]|AppDistribution.*not currently published|InAppMessaging.*not currently published" Readme.md docs/Firebase/NuGet samples/Firebase/AppCheck
• git diff --check
• git ls-remote --tags origin v12.6.0
• gh release view v12.6.0
• nuget.org flat-container checks for all 16 Firebase 12.6.0 packages plus GoogleAppMeasurement 12.6.0