fix: lower default streaming read timeout to 20s [AIR-1092]

[Cubestormer-IV]

Context

stream and stream_async in chat, fim, and agents inherited the global 300s timeout. For SSE streaming this becomes an httpx read timeout between chunks — a stalled stream hangs for 5 minutes before failing.

Linear: AIR-1092

Implementation

Changed the hardcoded fallback from 300000 to 20000 (20s) in stream/stream_async methods only. Non-streaming (complete/complete_async) stay at 300s.

The x-speakeasy-timeout extension was also added to the streaming operations in the dashboard OpenAPI spec (PR #41377), but local testing confirmed Speakeasy currently ignores per-operation x-speakeasy-timeout — all methods still fell back to 300000 after regeneration. This direct fix is needed until that's resolved upstream.

Timeout priority remains: per-call timeout_ms= > Mistral(timeout_ms=...) > per-operation default (now 20s for streaming).

Checks / QA

• Verify chat.stream(...), fim.stream(...), agents.stream(...) time out after ~20s of silence rather than 5 minutes
• Confirm chat.complete(...) still uses 300s default
• Override still works: client.chat.stream(..., timeout_ms=60000) should use 60s