feat: emit $is_server property on captured events

[turnipdabeets]

Adds $is_server: true to every event captured by this SDK so PostHog ingestion can identify server-side events.

[greptile-apps]

Comments Outside Diff (1)

1. test/posthog_test.exs, line 115-116 (link)

   The "ignores set context" test asserts $lib and $lib_version from @system_global_properties but was not updated to also assert "$is_server": true. Adding it here keeps the three system properties consistently checked in every test that already validates system properties.

   Prompt To Fix With AI

   This is a comment left during a code review.
   Path: test/posthog_test.exs
   Line: 115-116
   
   Comment:
   The "ignores set context" test asserts `$lib` and `$lib_version` from `@system_global_properties` but was not updated to also assert `"$is_server": true`. Adding it here keeps the three system properties consistently checked in every test that already validates system properties.
   
   How can I resolve this? If you propose a fix, please make it concise.

   Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
test/posthog_test.exs:115-116
The "ignores set context" test asserts `$lib` and `$lib_version` from `@system_global_properties` but was not updated to also assert `"$is_server": true`. Adding it here keeps the three system properties consistently checked in every test that already validates system properties.

_{Reviews (1): Last reviewed commit: "feat: emit $is_server property on captur..." | Re-trigger Greptile}

[github-actions]

posthog-elixir Compliance Report

Date: 2026-06-03 14:55:22 UTC
Duration: 107437ms

⚠️ Some Tests Failed

42/45 tests passed, 3 failed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	610ms
Format Validation.Event Has Uuid	✅	610ms
Format Validation.Event Has Lib Properties	✅	610ms
Format Validation.Distinct Id Is String	✅	610ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	609ms
Format Validation.Event Has Timestamp	✅	610ms
Retry Behavior.Retries On 503	✅	5616ms
Retry Behavior.Does Not Retry On 400	✅	2612ms
Retry Behavior.Does Not Retry On 401	✅	2613ms
Retry Behavior.Respects Retry After Header	✅	5615ms
Retry Behavior.Implements Backoff	✅	15627ms
Retry Behavior.Retries On 500	✅	5615ms
Retry Behavior.Retries On 502	✅	5617ms
Retry Behavior.Retries On 504	✅	5616ms
Retry Behavior.Max Retries Respected	✅	15626ms
Deduplication.Generates Unique Uuids	✅	621ms
Deduplication.Preserves Uuid On Retry	✅	5616ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10621ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	614ms
Compression.Sends Gzip When Enabled	✅	609ms
Batch Format.Uses Proper Batch Structure	✅	610ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	615ms
Error Handling.Does Not Retry On 403	✅	2613ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5616ms

Feature_Flags Tests

⚠️ 13/16 tests passed, 3 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	9ms
Request Payload.Flags Request Uses V2 Query Param	✅	7ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	8ms
Request Payload.Flags Request Omits Authorization Header	✅	7ms
Request Payload.Token In Flags Body Matches Init	✅	8ms
Request Payload.Groups Round Trip	✅	8ms
Request Payload.Groups Default To Empty Object	✅	8ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	7ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	8ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	7ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	8ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	608ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	12ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	7ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	610ms

Failures

request_payload.disable_geoip_omitted_defaults_to_false

Field 'geoip_disable' not found in /flags request body at path 'geoip_disable'. Available keys: ['groups', 'api_key', 'distinct_id', 'flag_keys_to_evaluate', 'group_properties', 'person_properties']

request_lifecycle.mock_response_value_is_returned_to_caller

Last action result missing field 'value'. Keys: ['error', 'success']

side_effect_events.get_feature_flag_captures_feature_flag_called_event

Expected 1 events with name '$feature_flag_called', got 0