[Bug] OAuth2 authentication makes HTTP requests with no timeout, hanging the client indefinitely

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/pulsar-client-cpp/issues) and found nothing similar.

### Version

pulsar-client-cpp v4.2.0 (also verified at master, commit 1d08b2b, 2026-07-21)
Reproduced through pulsar-client (Python) 3.13.0 from PyPI, which pins pulsar-cpp 4.2.0
via dependencies.yaml.
OS: Linux x86_64

### Minimal reproduce step

Requires no Pulsar broker and no real OAuth2 issuer. The script below starts a stub TCP
listener that accepts the connection and then never replies — the behaviour of a
black-holed or overloaded issuer.

pip install pulsar-client==3.13.0
python3 repro_oauth_timeout.py 100

Script attached. It configures the client with operation_timeout_seconds=5 and
connection_timeout_ms=5000, points issuer_url at the stub, and calls create_producer().

### What did you expect to see?

The OAuth2 flow fails within a bounded, ideally configurable, period — and at minimum
within the client's own configured timeouts.

### What did you see instead?

The call never returns. Observed output:

[*] pulsar-client 3.13.0
[*] operation_timeout_seconds=5, connection_timeout_ms=5000
[*] Calling create_producer(); watchdog set to 100s

[stub:issuer] accepted TCP connection, now stalling (will never respond)
still blocked after 15.0s (no client-side timeout has fired)
still blocked after 30.0s (no client-side timeout has fired)
still blocked after 45.0s (no client-side timeout has fired)
still blocked after 60.0s (no client-side timeout has fired)
still blocked after 75.0s (no client-side timeout has fired)
still blocked after 90.0s (no client-side timeout has fired)
still blocked after 105.0s (no client-side timeout has fired)

[RESULT] REPRODUCED. Still blocked in OAuth2 discovery after 105.0s.

Both configured client timeouts were exceeded by more than 20x with no sign of returning.
Neither operationTimeoutSeconds nor connectionTimeoutMs bounds the auth flow's HTTP calls.

### Anything else?

Root cause: CurlWrapper::Options::timeoutInSeconds defaults to 0 (lib/CurlWrapper.h:55) and
is applied unconditionally at lib/CurlWrapper.h:123:

curl_easy_setopt(handle_, CURLOPT_TIMEOUT, options.timeoutInSeconds);

CURLOPT_TIMEOUT of 0 means no timeout in libcurl. CURLOPT_CONNECTTIMEOUT is never set
anywhere in the repository, leaving only libcurl's 300s default for the connect phase and
nothing at all once connected.

Both OAuth2 call sites leave the default in place:

1. Issuer discovery — lib/auth/AuthOauth2.cc:277, in fetchTokenEndpoint(), passes a
default-constructed options object:
curl.get(wellKnownUrl, "Accept: application/json", {}, tlsContext);

2. Token endpoint — lib/auth/AuthOauth2.cc:336-339, in fetchOauth2Token(), constructs
options but sets only postFields.

This looks specific to the OAuth2 path. Other components set a timeout on the same wrapper:
- lib/HTTPLookupService.cc:229 -> options.timeoutInSeconds = lookupTimeoutInSeconds_;
- lib/auth/athenz/ZTSClient.cc:369 -> options.timeoutInSeconds = REQUEST_TIMEOUT;

Impact, as hit in production by a user running OAuth2 in Kubernetes: pods hang rather than
fail, so the problem stays invisible to health checks; and a transient issuer fault cannot
self-heal because recovery never starts.

Suggested fix: set a default timeout at both OAuth2 call sites following the existing
HTTPLookupService/ZTSClient pattern, and set CURLOPT_CONNECTTIMEOUT in CurlWrapper. Ideally
expose the value as an OAuth2 parameter.

[repro_oauth_timeout.py](https://github.com/user-attachments/files/31149391/repro_oauth_timeout.py)

### Are you willing to submit a PR?

- [x] I'm willing to submit a PR!

Comienza con lib/CurlWrapper.h, especialmente con el valor predeterminado de options y la configuración de CURLOPT_TIMEOUT; después inspecciona fetchTokenEndpoint() y fetchOauth2Token() en lib/auth/AuthOauth2.cc. Compara su gestión del timeout con lib/HTTPLookupService.cc y lib/auth/athenz/ZTSClient.cc, y ejecuta el repro adjunto contra el emisor stub. Se considera terminado cuando las llamadas HTTP de OAuth2 devuelvan el control dentro de un timeout acotado en lugar de quedar bloqueadas indefinidamente.

Escrito por el modelo de indexación a partir del texto del issue.