feat: Create GlobalHookManager.

[NeaguGeorgiana23]

This PR

• Implements GlobalHookManager as an independent, thread-safe singleton to store and manage global hooks using std::shared_mutex.
• Decouples global hook management from OpenFeatureAPI, mirroring the design pattern established by GlobalContextManager. This allows ClientAPI to access global hooks during flag evaluation without introducing a circular dependency between :client_api and :openfeature_api in Bazel.
• Updates OpenFeatureAPI to delegate AddHook, AddHooks, GetHooks, and Shutdown (clearing hooks) to GlobalHookManager::GetInstance().
• Ensures nullptr hook pointers are filtered out and registration order is strictly preserved.
• Adds comprehensive unit tests in test/global_hook_manager_test.cpp covering initial state, single/batch hook registration, order preservation, null filtering, hook clearing, and multithreaded reader/writer stress testing.

Related Issues

Fixes #133

Follow-up Tasks

• Implement the hook execution lifecycle pipeline (before, after, error, finally) inside ClientAPI::EvaluateFlag.
• Add e2e tests for the complete hook evaluation pipeline.

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📝 Walkthrough

Walkthrough

The change adds a thread-safe GlobalHookManager singleton. OpenFeatureAPI delegates hook operations to it, and new Bazel tests cover storage behavior and concurrent access.

Changes

Global hook management

Layer / File(s)	Summary
GlobalHookManager implementation openfeature/global_hook_manager.h, openfeature/global_hook_manager.cpp	Adds singleton access, thread-safe hook insertion, retrieval, null filtering, ordering, and clearing.
OpenFeatureAPI integration openfeature/openfeature_api.h, openfeature/openfeature_api.cpp, openfeature/BUILD	Removes local hook storage from OpenFeatureAPI, delegates hook operations to GlobalHookManager, and adds the library dependency.
GlobalHookManager validation test/global_hook_manager_test.cpp, test/BUILD	Adds tests for singleton identity, hook behavior, clearing, null filtering, ordering, and concurrent reads and writes.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant OpenFeatureAPI
  participant GlobalHookManager
  participant HookStorage
  OpenFeatureAPI->>GlobalHookManager: AddHook or AddHooks
  GlobalHookManager->>HookStorage: Store non-null hooks
  OpenFeatureAPI->>GlobalHookManager: GetHooks
  GlobalHookManager->>HookStorage: Copy hooks under shared lock
  OpenFeatureAPI->>GlobalHookManager: ClearHooks during Shutdown
  GlobalHookManager->>HookStorage: Clear stored hooks

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Merge Risk: 🔵 Low · up to 79ce0

The new concurrency test can intermittently fail despite correct hook-manager behavior, reducing build reliability until writer startup is synchronized.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 8.70% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 23 functions across 7 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: adding the GlobalHookManager singleton.
Description check	✅ Passed	The description accurately covers the GlobalHookManager implementation, OpenFeatureAPI integration, design objective, null filtering, ordering, tests, and follow-up work.
Linked Issues check	✅ Passed	Issue #133 requires a dedicated global hook manager that decouples global hook storage from OpenFeatureAPI and follows the GlobalContextManager pattern. GlobalHookManager provides a function-loc…
Out of Scope Changes check	✅ Passed	The changes stay within issue #133. The build integration, OpenFeatureAPI delegation, implementation, and unit tests directly support the dedicated manager and decoupling objective. No unrelated pro…

✨ Finishing Touches 💡 1

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Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
In `@test/global_hook_manager_test.cpp`:
- Line 134: Update the test’s writer synchronization so it signals completion
only after the writer performs its first AddHook, then begin the stress-test
duration; ensure the main thread waits for that signal before setting stop and
reaching the final GetHooks assertion.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: c7220ce6-8875-47c8-8355-ab1ed2ef15b3

📥 Commits

Reviewing files that changed from the base of the PR and between ed3a027 and 79ce008.

📒 Files selected for processing (7)

• openfeature/BUILD
• openfeature/global_hook_manager.cpp
• openfeature/global_hook_manager.h
• openfeature/openfeature_api.cpp
• openfeature/openfeature_api.h
• test/BUILD
• test/global_hook_manager_test.cpp

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