feat(storage): add fetch_policy configuration option

[erka]

Add configurable policy for handling remote git fetch failures during
startup. This allows Flipt to continue operating with stale local data
when the remote repository is temporarily unavailable.

Related #4907

[codecov]

Codecov Report

❌ Patch coverage is 60.46512% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.07%. Comparing base (63e2ff8) to head (19613b5).
⚠️ Report is 1 commits behind head on v2.

Files with missing lines	Patch %	Lines
internal/storage/environments/environments.go	0.00%	8 Missing and 1 partial ⚠️
internal/storage/git/repository.go	74.19%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v2    #4920      +/-   ##
==========================================
+ Coverage   59.01%   59.07%   +0.05%     
==========================================
  Files         135      135              
  Lines       16874    16908      +34     
==========================================
+ Hits         9959     9988      +29     
- Misses       6234     6239       +5     
  Partials      681      681              

Flag	Coverage Δ
integrationtests	33.82% <9.30%> (-0.06%)	⬇️
unittests	50.12% <60.46%> (+0.37%)	⬆️

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[Copilot]

Pull Request Overview

This PR adds a configurable remote_startup_fetch_policy option that controls how Flipt handles remote Git fetch failures during startup. The policy can be set to "required" (default) or "optional", allowing Flipt to continue operating with stale local data when the remote repository is temporarily unavailable.

Key changes:

• Added RemoteStartupFetchPolicy configuration field supporting "required" (default) and "optional" policies
• Implemented connection error detection logic to differentiate network issues from other errors
• Modified startup fetch logic to conditionally allow failures when policy is "optional" and local commits exist

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
magefile.go	Updated test command to run specific test instead of full suite
internal/storage/git/repository_test.go	Added comprehensive tests for new fetch policy behavior and connection error handling
internal/storage/git/repository.go	Implemented fetch policy logic, connection error detection, and configuration option
internal/storage/environments/environments.go	Integrated fetch policy into environment manager's repository initialization and fetch logic
internal/config/storage.go	Added RemoteStartupFetchPolicy field to storage configuration
config/flipt.schema.json	Added schema validation for remoteStartupFetchPolicy enum values
config/flipt.schema.cue	Added CUE schema definition with default value for remote_startup_fetch_policy

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[dosubot]

Documentation Updates

1 document(s) were updated by changes in this PR

• Flipt SCM Providers and Git Integration (View Changes)

^{How did I do? Any feedback?}  

[markphelps]

looks great! just some thoughts on naming things

[markphelps]

looks great! i'll likely create a release tomorrow