chore!: remove serialization helpers from the public API

[turnipdabeets]

💡 Motivation and Context

Stacked on #637 (v6).

toMap() on PostHogConfig, PostHogBootstrapConfig, PostHogLogsConfig, PostHogRageClickConfig, PostHogSessionReplayConfig, PostHogErrorTrackingConfig and PostHogExceptionStepsConfig, plus PostHogFeatureFlagResult.fromMap(), are the platform-channel wire format, not a data shape for app code. For example, toMap() writes sendFeatureFlagEvents for the sendFeatureFlagEvent field. Because they're public, they're in the API snapshot and we'd have to keep them stable. A GitHub code search turned up no external callers. v6 is the place to drop them.

What changed:

• The seven toMap() methods are now extensions, and fromMap is now a top-level featureFlagResultFromMap(). Both live in lib/src/utils/channel_serialization.dart, which lib/posthog_flutter.dart doesn't export. The map bodies are unchanged.
• posthog_flutter_io.dart, posthog_flutter_web.dart and the tests import that file.
• The PostHogFeatureFlagResult constructor stays public.
• Added a v6 changeset entry, and regenerated api/posthog_flutter.api.json. The only removals are the seven toMap entries and fromMap.

Judgment calls:

• Why not @internal? The snapshot tool keeps @internal members (Posthog.config and currentScreen are still listed). Making the methods _private doesn't work either, because they're called from other libraries. Extensions in a file that isn't exported take them out of the API surface.
• Private access: only PostHogLogsConfig.toMap() used a private member, the static helper _wholeSeconds. It moved into the extension with it. Every other field it reads, such as host, screenshotScale and screenshotCompressionQuality, has a public getter. posthog_config.dart no longer needs its util/logging.dart import. That import was only there for the bootstrap flag-type warning, which moved too.
• fromMap as a top-level function: a Dart extension can't add a static member that callers reach as PostHogFeatureFlagResult.fromMap. I also removed the redundant result == null check, since result is! Map already covers null.
• I added a new changeset file rather than editing v6-remove-5x-deprecations.md, so this doesn't conflict with the parallel platform opt-in/opt-out PR. That PR also regenerates the API snapshot, so whichever PR lands second needs to regenerate it again.

💚 How did you test it?

• flutter analyze: no issues. This covers posthog_flutter_web.dart.
• flutter test: all 781 tests passed. The existing toMap/fromMap tests now run against the internal versions.
• dart format --set-exit-if-changed ./: clean.
• scripts/check-api-dart.sh --update: grepping the snapshot for toMap, fromMap, Serialization and featureFlagResultFromMap returns 0 matches.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Made with Claude Code. Anna chose the approach (unexported extensions instead of @internal) after checking that the API snapshot keeps @internal members.

🤖 Generated with Claude Code

[github-actions]

posthog-flutter Compliance Report

Date: 2026-10-07 15:53:28 UTC
Duration: 96850ms

✅ All Tests Passed!

45/45 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	144ms
Format Validation.Event Has Uuid	✅	119ms
Format Validation.Event Has Lib Properties	✅	117ms
Format Validation.Distinct Id Is String	✅	114ms
Format Validation.Token Is Present	✅	114ms
Format Validation.Custom Properties Preserved	✅	117ms
Format Validation.Event Has Timestamp	✅	117ms
Retry Behavior.Retries On 503	✅	5334ms
Retry Behavior.Does Not Retry On 400	✅	2117ms
Retry Behavior.Does Not Retry On 401	✅	2117ms
Retry Behavior.Respects Retry After Header	✅	8126ms
Retry Behavior.Implements Backoff	✅	15446ms
Retry Behavior.Retries On 500	✅	5228ms
Retry Behavior.Retries On 502	✅	5226ms
Retry Behavior.Retries On 504	✅	5226ms
Retry Behavior.Max Retries Respected	✅	15446ms
Deduplication.Generates Unique Uuids	✅	125ms
Deduplication.Preserves Uuid On Retry	✅	5226ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10333ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5229ms
Deduplication.No Duplicate Events In Batch	✅	125ms
Deduplication.Different Events Have Different Uuids	✅	116ms
Compression.Sends Gzip When Enabled	✅	116ms
Batch Format.Uses Proper Batch Structure	✅	113ms
Batch Format.Flush With No Events Sends Nothing	✅	109ms
Batch Format.Multiple Events Batched Together	✅	125ms
Error Handling.Does Not Retry On 403	✅	2117ms
Error Handling.Does Not Retry On 413	✅	2117ms
Error Handling.Retries On 408	✅	5224ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	14ms
Request Payload.Flags Request Uses V2 Query Param	✅	9ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	10ms
Request Payload.Flags Request Omits Authorization Header	✅	10ms
Request Payload.Token In Flags Body Matches Init	✅	10ms
Request Payload.Groups Round Trip	✅	11ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	10ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	9ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	10ms
Request Lifecycle.No Flags Request On Init Alone	✅	5ms
Request Lifecycle.No Flags Request On Normal Capture	✅	113ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	16ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	115ms