Make EventSource callbacks async

[mattt]

This PR attempts to fix an apparent race condition indicated by a flakey integration test by making the onMessage callback async and awaiting that before proceeding with reconnection or completion.

It also adds a new maximumFinalizationEventCount property (default = 100) to prevent unbounded memory growth when handling events sent by the server during finalization.

[Copilot]

Pull Request Overview

This PR refactors the test suite and updates the EventSource API to make callbacks async. The main changes include:

• Test function naming: Removes "test" prefix from test function names across all test files, aligning with Swift Testing conventions
• Async callbacks: Changes EventSource callbacks (onOpen, onMessage, onError) from synchronous to async functions
• Event delivery: Adds explicit event delivery loop after parser.finish() to ensure pending events are delivered with a safety limit of 100 events
• Test organization: Reorganizes IntegrationTests.swift into clearer nested suites: EventSourceAPITests and URLSessionBytesAPITests

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Tests/EventSourceTests/ParserTests.swift	Renamed all test functions to remove "test" prefix for consistency with Swift Testing conventions
Tests/EventSourceTests/IntegrationTests.swift	Renamed tests, reorganized into nested suites, updated to use async callbacks, removed unused ErrorTracker actor
Tests/EventSourceTests/AsyncEventsSequenceTests.swift	Renamed all test functions to remove "test" prefix
Sources/EventSource/EventSource.swift	Changed callbacks to async, added event delivery loop after parser.finish(), removed some explanatory comments

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