Add StreamApiException, StreamRateLimitException, StreamTransportException,
and StreamTaskException as checked subclasses of the existing StreamException.
Carries the previously-dropped APIError fields (unrecoverable, details) and
parses the Retry-After header (RFC 7231: integer seconds + HTTP-date) on 429.
Transport-layer IO failures are now classified into a per-error-type enum and
the cause chain is preserved at every wrap point.

Adds client.waitForTask(taskId) as a customer-facing main-source helper that
polls until terminal. Failed tasks throw StreamTaskException populated from
ErrorResult; the wait elapsing throws StreamTransportException(errorType=
timeout). Removes the duplicate test-only waitForTask helper from ChatTestBase
and updates the 5 callers.

No breaking API changes: subclasses extend the existing checked StreamException,
getResponseData() continues to return a populated mirror on API errors, and
existing throws StreamException declarations continue to compile.

Auto-formatting via `./gradlew spotlessApply` over the CHA-2958 files.
No behavior change.

…heck

Two test-only fixes for CI on JDK 21:
1. `request()` helper now declares `throws StreamException` (StreamRequest's
   constructor is checked since CHA-2958's reshape).
2. `instanceof RuntimeException` against a statically-typed `StreamException`
   is an unconditional-pattern compile error on JDK 21; use
   `RuntimeException.class.isInstance(e)` to keep the runtime assertion.

CI sets `STREAM_BASE_URL: ${{ vars.STREAM_BASE_URL }}`, which renders to an
empty string when the GitHub variable is not configured. With the previous
`env.getOrDefault(..., System.getProperty(...))` shape, an empty env value
won and overrode any `io.getstream.url` system property set by tests, so
unit tests fell through to the real Stream API using the real credentials
also injected via env. Tests in `StreamWaitForTaskTest` hit the live
`get-task/t1` endpoint and got `StreamApiException: Can't find task with id`.

Now the env var falls through to the system property when it is null or
blank, so MockWebServer-backed tests work and any future deployment that
leaves STREAM_BASE_URL empty falls back to the SDK default base URL
instead of broken empty URLs.

CI sets STREAM_BASE_URL to the real Stream API URL, and the Properties-based
StreamHTTPClient constructor gives env vars priority over system properties.
The previous setUp set `io.getstream.url` via System.setProperty and then
called `new StreamSDKClient(System.getProperties())`, which still read the
real-URL env var and pointed every request at the live API. The mocked task
JSONs were never consumed and the SDK got back real "Can't find task with
id" responses.

Switched setUp to the credentials-only StreamHTTPClient constructor (which
does not read env vars) and used reflection to force the private `baseUrl`
field onto the MockWebServer URL. The SDK client is then wrapped via
`new StreamSDKClient(StreamHTTPClient)`. Removes the system-property dance
and the env precedence problem entirely.