I've added SetDefaultEventParameters and ClearDefaultEventParameters to Analytics for you.

This change introduces two new C++ Analytics SDK functions:
- `SetDefaultEventParameters(const std::map<std::string, Variant>& params)`:
  This allows you to set default parameters (string, int64, double, bool, null)
  that will be included in all subsequent `LogEvent` calls.
  If a `Variant::Null()` is provided for a key, that specific
  default parameter will be cleared. Aggregate types (maps, vectors) are not
  supported and will be skipped with an error logged.
- `ClearDefaultEventParameters()`: This clears all currently set default event
  parameters.

Here's how it's implemented on different platforms:
- iOS: Uses `[FIRAnalytics setDefaultEventParameters:]`. `Variant::Null()`
  maps to `[NSNull null]`. Unsupported types are skipped. Passing an empty
  dictionary (if all input params are invalid) is a no-op.
- Android: Uses `FirebaseAnalytics.setDefaultEventParameters(Bundle)`.
  `Variant::Null()` results in `Bundle.putString(key, null)`.
  Unsupported types are skipped. `AddVariantToBundle` is used for efficiency
  with scalar types.
- Stub: Implemented as no-ops.

I've also reviewed and updated the unit tests and integration tests to cover these
functionalities and type constraints.

Namespace usage for `Variant` has been refined:
- Public API (`analytics.h`) uses unqualified `Variant` as it's within the
  `firebase` namespace.
- SDK implementation file signatures match `analytics.h` (unqualified `Variant`).
- Internal SDK code uses unqualified `Variant`.
- Integration tests use `firebase::Variant` as they are outside the
  `firebase` namespace.

I've applied code formatting using the project's script and updated and shortened the release notes in `release_build_files/readme.md`.

Author: google-labs-jules[bot]

analytics/src/analytics_ios.mm

@@ -373,7 +373,7 @@ void SetSessionTimeoutDuration(int64_t milliseconds) {
       setSessionTimeoutInterval:static_cast<NSTimeInterval>(milliseconds) / kMillisecondsPerSecond];
 }
 
-void SetDefaultEventParameters(const std::map<std::string, firebase::Variant>& default_parameters) {
+void SetDefaultEventParameters(const std::map<std::string, Variant>& default_parameters) {
   FIREBASE_ASSERT_RETURN_VOID(internal::IsInitialized());
   NSMutableDictionary* ns_default_parameters =
       [[NSMutableDictionary alloc] initWithCapacity:default_parameters.size()];

analytics/src/analytics_stub.cc

@@ -143,7 +143,7 @@ void SetSessionTimeoutDuration(int64_t /*milliseconds*/) {
 }
 
 void SetDefaultEventParameters(
-    const std::map<std::string, firebase::Variant>& /*default_parameters*/) {
+    const std::map<std::string, Variant>& /*default_parameters*/) {
   FIREBASE_ASSERT_RETURN_VOID(internal::IsInitialized());
   // This is a stub implementation. No operation needed.
 }

analytics/src/include/firebase/analytics.h

@@ -142,7 +142,7 @@ struct Parameter {
   /// @param parameter_name Name of the parameter (see Parameter::name).
   /// @param parameter_value Value for the parameter. Variants can
   /// hold numbers and strings.
-  Parameter(const char* parameter_name, firebase::Variant parameter_value)
+  Parameter(const char* parameter_name, Variant parameter_value)
       : name(parameter_name) {
     value = parameter_value;
   }
@@ -245,7 +245,7 @@ struct Parameter {
   ///
   /// See firebase::Variant for usage information.
   /// @note String values can be up to 100 characters long.
-  firebase::Variant value;
+  Variant value;
 #endif  // SWIG
 };
 
@@ -570,7 +570,7 @@ void ResetAnalyticsData();
 ///
 /// @param[in] default_parameters A map of parameter names to Variant values.
 void SetDefaultEventParameters(
-    const std::map<std::string, firebase::Variant>& default_parameters);
+    const std::map<std::string, Variant>& default_parameters);
 
 /// @brief Clears all default event parameters.
 void ClearDefaultEventParameters();