Autogenerate document IDs

[danReynolds]

Given that this library takes some cues from Cloud Firestore, which supports automatically generating IDs, let's add that.

[Copilot]

Pull Request Overview

This PR removes the uuid package dependency and replaces it with a custom generateId() implementation that generates cryptographically secure, URL-safe random IDs using Dart's built-in Random.secure().

• Removed the uuid dependency from pubspec.yaml
• Implemented a custom ID generation function in lib/utils/id.dart
• Updated minimum SDK version from >=3.0.0 to >=3.3.0

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lib/utils/id.dart	New custom ID generator using Random.secure() with 21-character default length and 126 bits of entropy
lib/persistor/worker/messages.dart	Replaced uuid.v4() with generateId() for message request IDs
lib/broadcast_observer.dart	Replaced uuid.v4() with generateId() for observer IDs
lib/collection.dart	Made doc() ID parameter optional, using generateId() as default
lib/loon.dart	Replaced import of uuid package with custom id.dart utility, simplified type casting
example/lib/random_operation_runner.dart	Replaced uuid.v4() with generateId() for test user IDs
test/utils.dart	Removed unused uuid import and constant
pubspec.yaml	Removed uuid dependency and bumped minimum SDK version to >=3.3.0

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