feat: support starts_with and ends_with operators in local evaluation

[haacked]

💡 Motivation and Context

PostHog/posthog#72992 added four case-insensitive property filter operators — starts_with, not_starts_with, ends_with, not_ends_with — across HogQL, the flags service, and the UI. The UI surface is gated behind a feature flag until server-side SDKs can evaluate these operators locally; on SDK versions without support, flags using them fall back to remote evaluation.

This adds local evaluation support for the four operators, mirroring icontains: both sides are stringified and lowercased, and the not_ variants are exact negations. Filter values are single strings — server-side validation rejects list values for these operators, matching the flags service behavior.

💚 How did you test it?

New unit tests mirror the flags service's own test matrix for these operators: case-insensitive anchored matching, mid-string non-matches (prevalue does not match starts_with: "Val"), numeric stringification (323 matches starts_with: "3", 123 does not), negation inverses, and missing-key inconclusive behavior.

bundle exec rspec spec/posthog/feature_flag_spec.rb passes locally (106 examples), and bundle exec rubocop reports no offenses on the changed files.

📝 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

• Added a changeset file (minor, posthog-ruby package)

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with PostHog Code as part of rolling out local evaluation for these operators across all server-side SDKs. The implementation deliberately mirrors the existing icontains branch in this repo (same string coercion and case folding) and the reference behavior in the Rust flags service (rust/feature-flags/src/properties/property_matching.rs in PostHog/posthog). The changeset file was written by hand following the repo's existing format.

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Created with PostHog Code

[github-actions]

posthog-ruby Compliance Report

Date: 2026-07-31 02:44:16 UTC
Duration: 98478ms

✅ 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	✅	110ms
Format Validation.Event Has Uuid	✅	106ms
Format Validation.Event Has Lib Properties	✅	108ms
Format Validation.Distinct Id Is String	✅	107ms
Format Validation.Token Is Present	✅	107ms
Format Validation.Custom Properties Preserved	✅	106ms
Format Validation.Event Has Timestamp	✅	107ms
Retry Behavior.Retries On 503	✅	5314ms
Retry Behavior.Does Not Retry On 400	✅	2108ms
Retry Behavior.Does Not Retry On 401	✅	2109ms
Retry Behavior.Respects Retry After Header	✅	8116ms
Retry Behavior.Implements Backoff	✅	15722ms
Retry Behavior.Retries On 500	✅	5212ms
Retry Behavior.Retries On 502	✅	5211ms
Retry Behavior.Retries On 504	✅	5212ms
Retry Behavior.Max Retries Respected	✅	15522ms
Deduplication.Generates Unique Uuids	✅	111ms
Deduplication.Preserves Uuid On Retry	✅	5211ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10417ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5214ms
Deduplication.No Duplicate Events In Batch	✅	113ms
Deduplication.Different Events Have Different Uuids	✅	109ms
Compression.Sends Gzip When Enabled	✅	107ms
Batch Format.Uses Proper Batch Structure	✅	106ms
Batch Format.Flush With No Events Sends Nothing	✅	4ms
Batch Format.Multiple Events Batched Together	✅	112ms
Error Handling.Does Not Retry On 403	✅	2109ms
Error Handling.Does Not Retry On 413	✅	2108ms
Error Handling.Retries On 408	✅	5212ms

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	✅	106ms
Request Payload.Flags Request Omits Authorization Header	✅	107ms
Request Payload.Token In Flags Body Matches Init	✅	106ms
Request Payload.Groups Round Trip	✅	107ms
Request Payload.Groups Default To Empty Object	✅	107ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	107ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	108ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	109ms
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	✅	110ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	106ms
Retry Behavior.Retries Flags On 502	✅	209ms
Retry Behavior.Retries Flags On 504	✅	250ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	108ms