Add SetDefaultEventParameters and ClearDefaultEventParameters to Analytics

This change introduces two new C++ Analytics SDK functions:
- `SetDefaultEventParameters(const std::map<std::string, Variant>& params)`:
  Allows setting 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. Passing only invalid
  parameters resulting in an empty parameter set is a no-op on iOS/Android.
- `ClearDefaultEventParameters()`: Clears all currently set default event
  parameters.

Platform implementations:
- iOS: Uses `[FIRAnalytics setDefaultEventParameters:]`. `Variant::Null()`
  maps to `[NSNull null]`. Unsupported types are skipped.
- 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.

Unit tests and integration tests have been reviewed and updated. Integration
test names have been clarified.

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.

Code formatting applied using the project's script.
Release notes in `release_build_files/readme.md` updated and shortened.

Author: google-labs-jules[bot]

analytics/integration_test/src/integration_test.cc

@@ -341,7 +341,7 @@ TEST_F(FirebaseAnalyticsTest, TestSetConsent) {
   did_test_setconsent_ = true;
 }
 
-TEST_F(FirebaseAnalyticsTest, TestDefaultEventParametersUsage) {
+TEST_F(FirebaseAnalyticsTest, TestSetDefaultEventParameters) {
   LogInfo(
       "Testing SetDefaultEventParameters with initial values, then updating "
       "with Null.");
@@ -377,11 +377,11 @@ TEST_F(FirebaseAnalyticsTest, TestDefaultEventParametersUsage) {
   // Actual parameter presence on logged events would be verified by
   // backend/native tests.
   LogInfo(
-      "TestDefaultEventParametersUsage completed. Calls were made "
+      "TestSetDefaultEventParameters completed. Calls were made "
       "successfully.");
 }
 
-TEST_F(FirebaseAnalyticsTest, TestClearDefaultEventParametersFunctionality) {
+TEST_F(FirebaseAnalyticsTest, TestClearDefaultEventParameters) {
   LogInfo("Testing ClearDefaultEventParameters.");
 
   std::map<std::string, firebase::Variant> defaults_to_clear;
@@ -403,7 +403,7 @@ TEST_F(FirebaseAnalyticsTest, TestClearDefaultEventParametersFunctionality) {
   ProcessEvents(500);
 
   LogInfo(
-      "TestClearDefaultEventParametersFunctionality completed. Call was made "
+      "TestClearDefaultEventParameters completed. Call was made "
       "successfully.");
 }