Add AI rules for feature flags (#4819)

* Add scope based feature flags

* fix equals

* add serialization tests

* Create new reference on buffer clone

* add comment explaining the merge method

* optimize merge method

* make flag and result nullable params

* handle empty/noop buffer on merge and add tests for it

* optimize add method

* format; fix test

* Add rules for feature flags and mention in overview_dev

* Fix duplicate check

* Update sentry/src/main/java/io/sentry/SentryOptions.java

Co-authored-by: Lorenzo Cian <17258265+lcian@users.noreply.github.com>

* Format code

* changelog

* nullable getFeatureFlags

---------

Co-authored-by: Lorenzo Cian <17258265+lcian@users.noreply.github.com>
Co-authored-by: Sentry Github Bot <bot+github-bot@sentry.io>

Author: 3 people

.cursor/rules/feature_flags.mdc

@@ -0,0 +1,24 @@
+---
+alwaysApply: false
+description: Feature Flags
+---
+# Java SDK Feature Flags
+
+There is a scope based and a span based API for tracking feature flag evaluations.
+
+## Scope Based API
+
+The `addFeatureFlag` method can be used to track feature flag evaluations. It exists on `Sentry` static API as well as `IScopes` and `IScope`.
+
+The `maxFeatureFlags` option controls how many flags are tracked per scope and also how many are sent to Sentry as part of events.
+Scope based feature flags can also be disabled by setting the value to 0. Defaults to 100 feature flag evaluations.
+
+Order of feature flag evaluations is important as we only keep track of the last {maxFeatureFlag} items.
+
+When a feature flag evluation with the same name is added, the previous one is removed and the new one is stored so that it'll be dropped last.
+
+When sending out an error event, feature flag buffers from all three scope types (global, isolation and current scope) are merged, chosing the newest {maxFeatureFlag} entries across all scope types. Feature flags are sent as part of the `flags` context.
+
+## Span Based API
+
+tbd

.cursor/rules/overview_dev.mdc

@@ -34,6 +34,14 @@ Use the `fetch_rules` tool to include these rules when working on specific areas
   - Rate limiting, cache rotation
   - Android vs JVM caching differences
 
+- **`feature_flags`**: Use when working with:
+  - Feature flag tracking and evaluation
+  - `addFeatureFlag()`, `getFeatureFlags()` methods
+  - `FeatureFlagBuffer`, `FeatureFlag` protocol
+  - `maxFeatureFlags` option and buffer management
+  - Feature flag merging across scope types
+  - Scope-based vs span-based feature flag APIs
+
 ### Integration & Infrastructure
 - **`opentelemetry`**: Use when working with:
   - OpenTelemetry modules (`sentry-opentelemetry-*`)
@@ -63,3 +71,4 @@ Use the `fetch_rules` tool to include these rules when working on specific areas
    - new module/integration/sample → `new_module`
    - Cache/offline/network → `offline`
    - System test/e2e/sample → `e2e_tests`
+   - Feature flag/addFeatureFlag/flag evaluation → `feature_flags`