Add telemetry metrics for sandbox usage

[stuartc]

Description

This PR adds Prometheus telemetry metrics to track sandbox project usage. The new SandboxPromExPlugin module emits counters for:

sandbox_merged_event_count
description: "Count of sandbox projects merged into targets."

sandbox_deleted_event_count
description: "Count of sandbox projects manually deleted."

workflow_saved_event_count
tags: [:is_sandbox],
  description: "Count of workflow saves, tagged by project type."

provisioner_import_event_count
  tags: [:is_sandbox],
  description: "Count of provisioner imports, tagged by project type."

Closes #4101

Validation steps

1. Run mix test - all tests pass including new telemetry tests
2. Check Prometheus metrics endpoint includes new sandbox counters
3. Create/merge/delete a sandbox and verify counters increment

Additional notes for the reviewer

1. There are no seed events for these metrics based on the understanding that the very first event will be dropped but only once in the lifespan of the Prometheus series (when using GMP). This should be the lessor of two evils, the other situation being seeding and having an extra count every time the server starts.
2. Made ObanManagerTest synchronous to fix a flaky test unrelated to this feature
3. Removed unused socket variable from two workflow channel tests

AI Usage

• I have used Claude Code

Pre-submission checklist

• I have performed an AI review of my code
• I have implemented and tested all related authorization policies
• I have updated the changelog
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.30%. Comparing base (b5d6c75) to head (9aa970b).

Files with missing lines	Patch %	Lines
lib/lightning/projects/sandboxes.ex	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4342      +/-   ##
==========================================
+ Coverage   89.21%   89.30%   +0.09%     
==========================================
  Files         425      426       +1     
  Lines       20011    20024      +13     
==========================================
+ Hits        17852    17882      +30     
+ Misses       2159     2142      -17     

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