fix: mirror rails request user agent into $raw_user_agent

[lricoy]

💡 Motivation and Context

posthog-rails attaches the request's User-Agent to events as $user_agent, but PostHog's server-side classification — web analytics bot detection ($virt_is_bot and friends) and the materialized-column fast path — keys on the standardized $raw_user_agent property (the one posthog-js sends). Server-side-tracked pageviews forwarding a real client UA are exactly the traffic where bot classification matters, and today they miss it.

PostHog/posthog#68253 removes the $user_agent query-time fallback for performance (it forced a full properties-blob read on every classification query — see the PR for production measurements), so this change mirrors the header into $raw_user_agent, aligning posthog-rails with the standardized property. Related: PostHog/posthog#68232.

$user_agent is still sent for backwards compatibility.

💚 How did you test it?

Agent-authored. Extended the request-context specs to assert $raw_user_agent carries the same value in all four scenarios that assert $user_agent; relying on CI to run rspec (no compatible local ruby toolchain in this session).

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• Changeset added for posthog-rails.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Built with Claude Code as part of a cross-repo effort to standardize on $raw_user_agent (PostHog/posthog#68232, PostHog/posthog#68253, with sibling PRs to posthog-android and posthog-python).

[greptile-apps]

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[github-actions]

posthog-ruby Compliance Report

Date: 2026-07-03 15:26:53 UTC
Duration: 97975ms

✅ All Tests Passed!

46/46 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	106ms
Format Validation.Event Has Uuid	✅	105ms
Format Validation.Event Has Lib Properties	✅	108ms
Format Validation.Distinct Id Is String	✅	106ms
Format Validation.Token Is Present	✅	105ms
Format Validation.Custom Properties Preserved	✅	106ms
Format Validation.Event Has Timestamp	✅	106ms
Retry Behavior.Retries On 503	✅	5312ms
Retry Behavior.Does Not Retry On 400	✅	2108ms
Retry Behavior.Does Not Retry On 401	✅	2108ms
Retry Behavior.Respects Retry After Header	✅	8014ms
Retry Behavior.Implements Backoff	✅	15621ms
Retry Behavior.Retries On 500	✅	5211ms
Retry Behavior.Retries On 502	✅	5211ms
Retry Behavior.Retries On 504	✅	5210ms
Retry Behavior.Max Retries Respected	✅	15517ms
Deduplication.Generates Unique Uuids	✅	109ms
Deduplication.Preserves Uuid On Retry	✅	5211ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10315ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5213ms
Deduplication.No Duplicate Events In Batch	✅	109ms
Deduplication.Different Events Have Different Uuids	✅	106ms
Compression.Sends Gzip When Enabled	✅	105ms
Batch Format.Uses Proper Batch Structure	✅	105ms
Batch Format.Flush With No Events Sends Nothing	✅	4ms
Batch Format.Multiple Events Batched Together	✅	108ms
Error Handling.Does Not Retry On 403	✅	2107ms
Error Handling.Does Not Retry On 413	✅	2108ms
Error Handling.Retries On 408	✅	5210ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	108ms
Request Payload.Flags Request Uses V2 Query Param	✅	107ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	108ms
Request Payload.Flags Request Omits Authorization Header	✅	105ms
Request Payload.Token In Flags Body Matches Init	✅	106ms
Request Payload.Groups Round Trip	✅	106ms
Request Payload.Groups Default To Empty Object	✅	106ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	105ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	106ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	106ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	105ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	109ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	104ms
Retry Behavior.Retries Flags On 502	✅	209ms
Retry Behavior.Retries Flags On 504	✅	208ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	108ms