feat: add isClosed observer to TestSseClient

[kinyoklion]

Adds a one-line, additive bool get isClosed to TestSseClient so test code can assert that the system under test correctly calls close() on its SSE client.

```dart
/// Whether [close] has been called on this client. Test-only --
/// production [SSEClient] implementations do not expose this state,
/// and tests asserting against it are inherently white-box. Use to
/// verify that code under test correctly tears the connection down.
bool get isClosed => _messageEventsController.isClosed;
```

Why this is its own PR

Cleaner changelog: this is a feat: for the launchdarkly_event_source_client package; the FDv2 streaming work that consumes it (PR #267) is feat: for launchdarkly_common_client. Splitting them lets release-please attribute each correctly.

Why test-only is OK on a public API

The TestSseClient class is already documented as test-only:

An SSE client to use for testing.

Changes may be made to this class without following semantic conventions.

The new getter inherits that contract; the docstring on isClosed reiterates it explicitly so a reader of the getter alone has the context.

Verification

dart format, dart analyze lib test, and the full event_source_client test suite (46 tests) all pass.

---

Note

Low Risk
Low risk additive change limited to the test-only TestSseClient API. No production SSE transport logic or data handling is modified.

Overview
Adds a test-only isClosed getter to TestSseClient so unit tests can assert that code under test properly calls close() and tears down the SSE stream.

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