fix: don't make a recoverable connection drop look like a crash

[JulienEllie]

What

A connection dropped mid-stream (httpx.ReadError) escaped the streaming
retry classifier and bubbled all the way up to the interactive REPL, which
dumped a ~60-line traceback. The loop technically survived, but the wall of
stack frames made a transient VPN/WiFi blip look like a hard crash.

Why it happened

_RETRYABLE_EXCEPTIONS enumerated individual transport error types:

_RETRYABLE_EXCEPTIONS = (
    httpx.RemoteProtocolError,
    httpx.ReadTimeout,
    httpcore.RemoteProtocolError,
)

httpx.ReadError — the error you get when a socket dies mid-response — wasn't
in the list, so should_retry_streaming() returned False and the error
propagated instead of being retried. Classic whack-a-mole: one missing mole.

The fix

1. Broaden the retry net to umbrella base classes. httpx.NetworkError
   is the parent of ReadError, WriteError, ConnectError, and
   CloseError; httpx.TimeoutException covers every *Timeout variant.
   Listing the bases (plus the httpcore twins) means all transient
   transport failures get the existing 3x backoff retry — no more chasing
   subclasses one funeral at a time.

2. Stop scaring users with raw tracebacks for environment hiccups. New
   _render_turn_exception() helper: transient/connection errors get a
   friendly one-liner ("connection blipped, re-run your prompt, your session
   is intact"); genuine bugs still get the full, debuggable traceback. It
   reuses the same retry classifier so the retry path and the render path
   can never drift out of sync (DRY).

Tests

• Regression coverage in test_streaming_retry.py for ReadError,
  ConnectError, and httpcore.ReadError in the classifier.
• New test_cli_runner_turn_exception.py pins the friendly-message vs.
  full-traceback behavior of _render_turn_exception.
• 43 new/related tests pass; existing cli_runner suites (45) stay green.

Behavior change

• Before: dropped socket → traceback dump that looks like a crash.
• After: dropped socket → silently retried 3x; if it still can't recover,
  a single friendly line, REPL keeps going.