fix(firestore): prevent adding telemetry events to ended spans (#8847)

[quirogas]

In REST-mode streaming operations (such as getAll(), Query.get(), or DocumentReference.set()), DocumentReader resolves its operation Promise as soon
as response data is processed, which causes startActiveSpan() to close the OpenTelemetry span.

However, the underlying stream's 'end' event fires on a subsequent tick of the Node.js event loop:

resultStream.on('end', () => {
this._traceUtil.currentSpan().addEvent(Firestore.${methodName}: Completed, ...);
});

Because the span is already closed by the time this 'end' listener runs, calling addEvent() throws an error in modern OpenTelemetry runtimes.
Guarding addEvent() ensures late stream events return safely without interrupting execution.

• Added an isRecording() check in Span.addEvent() (dev/src/telemetry/span.ts) using this.span?.isRecording?.() === false to return early if a span is
  closed.
• Added a unit test in dev/test/tracing.ts to verify that Span.addEvent() exits safely without throwing when called on an ended span.

Context

OpenTelemetry Version Changes: In older versions of @opentelemetry/api (< 1.9.0), calling addEvent() on a closed span was silently ignored by the
trace SDK. Starting in @opentelemetry/api >= 1.9.0, OpenTelemetry introduced strict runtime checks (SpanImpl._isSpanEnded) that throw Error:
Operation attempted on ended Span.

• Pre-Existing Flakiness: This stream completion timing sequence was already present in @google-cloud/firestore@8.7.0 and caused open flakybot test
  failures (e.g., Tracing Tests E2E NON-GLOBAL-OTEL REST: query get() failed #8847, Tracing Tests E2E GLOBAL-OTEL GRPC: aggregate query get() failed #8910, Tracing Tests E2E NON-GLOBAL-OTEL REST: document reference set() failed #8867). Similar span lifecycle race conditions were previously resolved in Spanner (see logs of speech data sent in google-cloud-node #2184, please support customized jobId in BigQuery jobReference #2121) and Logging (datastore.get() with > 300 keys fails #1497).
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[gemini-code-assist]

Code Review

This pull request updates the addEvent method in the Span class to return early if the underlying span is no longer recording (isRecording() === false), preventing errors when adding events to ended spans. A unit test has also been added to verify that calling addEvent on an ended span does not throw an exception. There are no review comments, and I have no feedback to provide.