chore: remove unused dead code

[duriantaco]

Summary

• Removed 10 unused symbols identified via static analysis using Skylos (zero references across entire codebase, verified with grep)
• 10 files changed, 56 deletions, 0 additions

Changes

Symbol	File	Reason
WebhookSendError	webhooks/exceptions.py (deleted)	Never raised or caught
WebhookURLSerializer	webhooks/serializers.py	Never used in any view
ViewResponseDoesNotHaveStatus	sse/exceptions.py	Never raised or caught
ImproperlyConfiguredError	app/exceptions.py (deleted)	Never raised or imported
IdentitySerializerFull	environments/identities/serializers.py	Never used in any view
BaseDetailedPermissionsSerializer	permissions/serializers.py	Never imported
UTMDataModel	users/models.py	Never instantiated
DispatchResponse	custom_auth/mfa/trench/responses.py	Never returned
OAuthError	custom_auth/oauth/exceptions.py	Never raised (GithubError/GoogleError kept)
get_next_segment_priority	features/models.py	Never called

Also removes now-unused imports (EnvironmentSerializerFull, FeatureStateSerializerFull, pydantic.BaseModel).

Test plan

• Each symbol has zero references in the entire repo (confirmed via grep)
• All modified files pass syntax checks
• No new broken imports introduced

Found with Skylos — dead code detection for Python.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.25%. Comparing base (c2bddaf) to head (a2b7b4e).
⚠️ Report is 2 commits behind head on chore/remove-dead-code.

Additional details and impacted files

@@                    Coverage Diff                     @@
##           chore/remove-dead-code    #6953      +/-   ##
==========================================================
- Coverage                   98.31%   98.25%   -0.06%     
==========================================================
  Files                        1335     1333       -2     
  Lines                       49745    49583     -162     
==========================================================
- Hits                        48907    48718     -189     
- Misses                        838      865      +27     

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
flagsmith-frontend-staging	Ignored	Preview	Mar 16, 2026 6:26am

[matthewelwell]

Thanks for the contribution @duriantaco! I'm going to merge this into a branch in our own repository so that the currently failing CI checks pass can be run. It might be that some of this code is required by some of our private packages which are hosted in private repositories and installed at build time.

[matthewelwell]

I have opened #6955 which you can follow.