[PM-28339] Move FetchedResultsSubscription's initial fetch into background

[matt-livefront]

🎟️ Tracking

PM-28339

📔 Objective

When FetchedResultsSubscription is initialized, it performs an initial fetch of the fetch request. If the subscription is created on the main thread, which is often the case when subscribing to data in a processor, this occurs on the main thread. The fetch itself occurs on a background context (#2165), but performFetch() is synchronous and waits for the results. After adding the serial queue in the previous PR, this is an easy thing to also move into the background.

Overall Summary:

Metric	Baseline	After Change	Δ (%)
Hitch Duration (ms)	150.5	86.7	-43%
Avg Hitch Duration (ms)	44.6	34.7	-22%
Max Hitch Duration (ms)	80	60	-25%

Run Details

Baseline:

Duration (ms)	Average (ms)	Max (ms)
291.7	58.3	133.3
65.7	32.9	50
183.4	61.1	100
133.3	44.5	66.7
78.3	26.1	50

After Change

Duration (ms)	Average (ms)	Max (ms)
91.7	30.6	66.7
91.7	22.9	50
91.7	22.9	50
66.7	66.7	66.7
91.7	30.6	66.7

This was a result of profiling 5 runs before and after on startup on an iPhone 13 Pro.

⏰ 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

[claude]

Claude finished @matt-livefront's task in 2m 10s —— View job

---

Code Review Complete

• Analyzing PR changes
• Running code review
• Posting findings

Review Summary: Changes look good overall. The PR successfully moves the blocking performFetch() operation to a background context, achieving significant performance improvements (43% reduction in hitch duration). The implementation properly serializes access through the queue and handles errors consistently.

The bitwarden-code-reviewer agent has posted detailed inline comments and a summary. Please review those for specific feedback.

[github-actions]

Checkmarx One – Scan Summary & Details – 7acbae50-e2da-4966-b8f6-df2a579e16d8

Great job! No new security vulnerabilities introduced in this pull request

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.23%. Comparing base (c3d3862) to head (11b38ee).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/Platform/Utilities/FetchedResultsPublisher.swift	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2193      +/-   ##
==========================================
- Coverage   85.50%   84.23%   -1.27%     
==========================================
  Files        1740     1978     +238     
  Lines      147301   162364   +15063     
==========================================
+ Hits       125943   136771   +10828     
- Misses      21358    25593    +4235     

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