chore(flags): remove dead FlagError variants

[haacked]

Problem

rust/feature-flags/src/api/errors.rs has accumulated FlagError variants that are never constructed anywhere in the codebase. They exist only in the enum definition, the error_metadata() / is_5xx() / IntoResponse arms, and test matrices. They add boilerplate flagged in #30174 without carrying any runtime meaning.

Part of #30174.

Note

This re-opens @ricardothe3rd's contribution from #55115 on an internal branch. Fork PRs can't read the Depot build token, so the build-images jobs fail with permission_denied before any code is validated. Running from an internal branch lets the full CI suite execute. The change is entirely @ricardothe3rd's work and their commits are preserved unmodified.

Changes

Removes 5 variants that a full-workspace grep and LSP check confirmed have zero construction sites outside errors.rs and its test modules:

• DeserializeFiltersError
• NoGroupTypeMappings
• GroupTypeMappingFetchFailed
• PropertiesNotInCache
• StaticCohortMatchesNotCached

Also removes each variant's arm in error_metadata(), is_5xx(), and IntoResponse, the #[case] entries in the parameterized canonical_log.rs test, and the test vectors in errors.rs.

Net 77 deletions, 1 insertion across two files. There is no behavior change because the deleted code paths were unreachable in production.

Scoping note

This is preparatory cleanup for #30174. It does not yet introduce the InternalError(anyhow::Error) / Unavailable(anyhow::Error) buckets. That collapse is left to a follow-up so this PR stays a clean, low-risk, independently mergeable first step.

How did you test this code?

Verification from the original contribution, covering the cargo gates rather than a running service:

• cargo check -p feature-flags, clean compile
• cargo test -p feature-flags --lib api::errors, 14/14 pass
• cargo test -p feature-flags --lib handler::canonical_log, 78/78 pass
• cargo clippy -p feature-flags --all-targets, no new warnings
• cargo fmt -p feature-flags --check, clean
• Workspace-wide grep for each deleted variant name and its error_code() string, zero references outside the two edited files

Integration tests are left to CI, which has the pre-configured Django and ClickHouse environment.

Automatic notifications

• Publish to changelog? no
• Alert Sales and Marketing teams? no

Docs update

No docs changes needed.

Co-authored-by: Ricardo rarganaiii@gmail.com

Fixes: #55115

[github-actions]

Hey @haacked! 👋

It looks like your git author email on this PR isn't your @posthog.com address (haacked@gmail.com). Since you're on the PostHog team, it's worth pointing your local git author email at your @posthog.com address. Why it matters:

• Consistent work identity in git history — internal tooling that attributes commits to team members keys off your @posthog.com address.
• Keeps team contributions easy to tell apart from external community ones when scanning history.

You can fix it for this repo with:

git config user.email "you@posthog.com"

Or set it globally with git config --global user.email "you@posthog.com". No need to redo this PR — just a nudge for next time. 🙂

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Merge branch 'master' into fix/flag-erro..." | Re-trigger Greptile}

[Copilot]

Pull request overview

This PR cleans up the feature-flags Rust service error surface by removing FlagError enum variants that were no longer constructible anywhere in the repository, along with their now-unreachable handling and associated test matrix entries. This reduces boilerplate in preparation for the broader error-handling refactor described in #30174.

Changes:

• Removed five dead FlagError variants and their match arms in error_metadata() / IntoResponse, plus related test vectors.
• Updated canonical logging tests and error test matrices to reflect the reduced variant set (and to include HashKeyOverrideError in the canonical log test matrix).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rust/feature-flags/src/handler/canonical_log.rs	Updates the parameterized canonical log test cases to remove deleted errors and include HashKeyOverrideError.
rust/feature-flags/src/api/errors.rs	Removes dead FlagError variants and their response/metadata handling; updates test vectors accordingly.

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[gustavohstrassburger]

Trust stamp

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-06-01 21:42 UTC	Run
prod-us	✅ Deployed	2026-06-02 10:18 UTC	Run
prod-eu	✅ Deployed	2026-06-02 10:20 UTC	Run