feat(rpc): add bounded retry for transient GET failures

[randileeharper]

Summary

Closes #85.

CiderRpcClient._request made a single HTTP attempt per Cider call, so transient local failures (Cider restarting, a brief network hiccup) turned into user-visible errors even for idempotent reads like GET playback status. This adds a small, bounded retry for transient failures on safe read methods.

Behavior

• GET requests are retried on transient failures (connection errors and 5xx server errors) using exponential backoff (0.1 * 2**attempt), up to cider_retry_count retries (default 2, for 3 total attempts).
• POST writes are never retried. Cider playback POSTs (play, next, volume, queue) and run_amapi_v3 POSTs (including catalog/library searches) are stateful and not known to be idempotent, so a retry could duplicate a side effect.
• 4xx client errors are never retried — they are definitive and retrying would mask a genuine bad request.
• The failure_callback now fires once after retries are exhausted, rather than per attempt, so a transient blip doesn't flood failure reports.

Implementation

This mirrors the existing retry pattern in vesper/historian.py (HttpHistorianSink):

• New cider_retry_count: int = Field(default=2, ge=0) setting in Settings, surfaced in sanitized(), analogous to historian_retry_count.
• CiderRpcClient.__init__ takes an injectable sleep: Callable[[float], None] (defaults to time.sleep) so tests run without real delays.
• _request wraps a single-attempt helper _send_once. A private _TransientRequestError signals retriable failures (connection errors + 5xx); definitive failures (4xx, non-JSON) raise CiderRpcError immediately from _send_once.

Test commands run

.venv/bin/python -m pytest -q                  # 270 passed (was 256)
.venv/bin/python -m ruff check vesper tests   # All checks passed
.venv/bin/python -m mypy vesper               # Success: no issues found in 37 source files

13 new tests in tests/test_rpc.py cover: GET retry on connection error / 5xx then success, retry exhaustion, exponential backoff timing, POST never retried (connection error and 5xx), 4xx never retried, 204 handling, single failure-callback invocation after exhaustion, and cider_retry_count=0 disabling retries.