test(agent-stream): add timeouts to stream fixture harness

[legenddcr]

Summary

• add request and processing timeouts to the shared stream fixture harness
• fail fast with provider and fixture context when a stream fixture stalls
• add a regression test that proves stalled fixture processing times out instead of hanging

Verification

• cargo test -p bitfun-agent-stream --test stream_test_harness -- --nocapture
• cargo test -p bitfun-agent-stream --test stream_processor_openai -- --nocapture
• cargo test -p bitfun-agent-stream --test stream_processor_anthropic -- --nocapture

[legenddcr]

@limityan