🪲 BUG-#79: Preserve partial stream text on mid-response timeout

[FernandoCelmer]

Summary

• _stream() in GenericProvider no longer discards accumulated text/tool_calls when the socket times out mid-read — returns a partial ProviderResponse with stop_reason="connection_lost" instead, so the turn still gets persisted via session.add_assistant().
• Re-raises when nothing was streamed yet, so Agent._complete()'s existing retry logic still applies to a pure connection failure.
• Bumped the default/example provider timeout from 60s to 180s — too short for models with long reasoning.

Closes #79

Test plan

• pytest tests/providers/test_generic.py — 31 passed, including new regression tests for mid-stream timeout (partial preserved) and pure-timeout-with-nothing-streamed (still raises)
• pytest tests/ (excluding pre-existing flaky test_serve_end_to_end.py) — 326 passed

[FernandoCelmer]

Closing — consolidating into a single PR for #79 and #80 together.