[unified_analytics] Send enabled_features as an event parameter rather than a user property

[andrewkolos]

Fixes flutter/flutter#147327.

Summary. The Analytics constructor accepts a list of "enabled features" for the current user of the whatever tool is in use (src). For example, in the Flutter tool, this will be a comma-delimited list of flutter config items set to true (explicitly by the user). When any event is sent off to Google Analytics, this list will be included as a user property with the key "enabled_features".

See the parent issue. This list is prone to getting truncated due to the low character limit of user property values. This PR instead sends them as event properties. This has a much higher character limit of 500, which is hopefully sufficient (even if it is not, it would be a huge improvement over the current limit of 36, which makes collecting enabled features near-useless).

However, this creates a pitfall. If any new Event decides to include an enabled_features event property (and also passes a non-null argument for the enabledFeatures parameter in the Analytics constructor). There are a few ways handle this:

1. Do nothing. The likelihood of this happening is extremely low (but it will be quite confusing if it does happen).
2. Create an Event._ constructor that throws an exception if the eventData map includes an entry with a key of enabled_features. Re-route all of the existing constructors to utilize this one. This working depends on newly added events following the pattern of using Event._ instead of doing initialization themselves; but I suspect folks adding new events will naturally use the existing ones for inspiration, so this is fine to me.
3. Similar to 2, add this validation logic but instead do so in generateRequestBody. This keeps the validation logic much closer to the logic that inserts enabled_features, so it will make sense to first-time maintainers slightly more quickly. This also would require the least work to implement. The downside is that no exception would appear until send is called, and that exception would be thrown asynchronously.

I suspect 2 is probably the way to go, so I went ahead and implemented that. Let me know if you have a differing opinion. I made this is a separate commit, so it would be easy to revert.

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PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
unified_analytics	Breaking	7.0.1	7.0.2	8.0.0 Got "7.0.2" expected >= "8.0.0" (breaking changes)	⚠️

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Coverage ⚠️

File	Coverage
pkgs/unified_analytics/lib/src/analytics.dart	💚 72 % ⬆️ 0 %
pkgs/unified_analytics/lib/src/constants.dart	💔 Not covered
pkgs/unified_analytics/lib/src/event.dart	💔 98 % ⬇️ 1 %
pkgs/unified_analytics/lib/src/user_property.dart	💔 96 % ⬇️ 0 %
pkgs/unified_analytics/lib/src/utils.dart	💚 88 % ⬆️ 0 %

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unified_analytics	Condition PersistedSurvey GAClient UserProperty

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pkgs/clock/test/clock_test.dart
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pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
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pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/pubspec_parse/test/git_uri_test.dart
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PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
unified_analytics	Breaking	7.0.1	7.0.1	8.0.0 Got "7.0.1" expected >= "8.0.0" (breaking changes)	⚠️

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Changelog Entry ❗

Package	Changed Files
package:unified_analytics	pkgs/unified_analytics/lib/src/analytics.dart pkgs/unified_analytics/lib/src/event.dart pkgs/unified_analytics/lib/src/user_property.dart pkgs/unified_analytics/lib/src/utils.dart

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pkgs/unified_analytics/lib/src/analytics.dart	💚 72 % ⬆️ 0 %
pkgs/unified_analytics/lib/src/event.dart	💚 98 %
pkgs/unified_analytics/lib/src/user_property.dart	💔 96 % ⬇️ 0 %
pkgs/unified_analytics/lib/src/utils.dart	💚 88 % ⬆️ 0 %

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
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pkgs/bazel_worker/benchmark/benchmark.dart
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pkgs/benchmark_harness/integration_test/perf_benchmark_test.dart
pkgs/boolean_selector/example/example.dart
pkgs/clock/lib/clock.dart
pkgs/clock/lib/src/clock.dart
pkgs/clock/lib/src/default.dart
pkgs/clock/lib/src/stopwatch.dart
pkgs/clock/lib/src/utils.dart
pkgs/clock/test/clock_test.dart
pkgs/clock/test/default_test.dart
pkgs/clock/test/stopwatch_test.dart
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pkgs/html/example/main.dart
pkgs/html/lib/dom.dart
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pkgs/html/lib/html_escape.dart
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pkgs/html/lib/src/constants.dart
pkgs/html/lib/src/encoding_parser.dart
pkgs/html/lib/src/html_input_stream.dart
pkgs/html/lib/src/list_proxy.dart
pkgs/html/lib/src/query_selector.dart
pkgs/html/lib/src/token.dart
pkgs/html/lib/src/tokenizer.dart
pkgs/html/lib/src/treebuilder.dart
pkgs/html/lib/src/utils.dart
pkgs/html/test/dom_test.dart
pkgs/html/test/parser_feature_test.dart
pkgs/html/test/parser_test.dart
pkgs/html/test/query_selector_test.dart
pkgs/html/test/selectors/level1_baseline_test.dart
pkgs/html/test/selectors/level1_lib.dart
pkgs/html/test/selectors/selectors.dart
pkgs/html/test/support.dart
pkgs/html/test/tokenizer_test.dart
pkgs/pubspec_parse/test/git_uri_test.dart
pkgs/stack_trace/example/example.dart
pkgs/watcher/test/custom_watcher_factory_test.dart
pkgs/yaml_edit/example/example.dart

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Package	Version	Status	Publish tag (post-merge)
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package:clock	1.1.2	already published at pub.dev
package:code_builder	4.10.1	already published at pub.dev
package:coverage	1.11.1	already published at pub.dev
package:csslib	1.0.2	already published at pub.dev
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package:pool	1.5.2-wip	WIP (no publish necessary)
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package:unified_analytics	7.0.2	ready to publish	unified_analytics-v7.0.2
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[bkonyi]

This is still a draft. Is it ready for review?

[bkonyi]

LGTM! Thanks!