[PM-11133] Update Networking module for strict concurrency

[KatherineInCode]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-11133

📔 Objective

This is part of the iOS 18 / Xcode 16 / Swift 6 effort

This updates our Networking module to have no warnings with strict concurrency checking turned on, and also annotates Sendable to eliminate warnings in the main project.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

[github-actions[bot]]

Checkmarx One – Scan Summary & Details – 557e1e9a-e957-467e-8ff3-58a0470b94ba

No New Or Fixed Issues Found

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.50%. Comparing base (ffbd89a) to head (ca562a8). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #851 +/- ## ======================================= Coverage 88.50% 88.50% ======================================= Files 609 609 Lines 30303 30303 ======================================= Hits 26820 26820 Misses 3483 3483 ```

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