fix(throttle): close the request body on throttle errors and log at debug

[adamwoolhether]

The throttle round tripper now closes the request body when it fails a request. It also logs at Debug level instead of Info.

Changes

• RoundTrip closes r.Body when the early context check, the limiter wait, or the post-wait context check fails. The http.RoundTripper contract requires the transport to close the body on errors. http.Client does not close it after a transport error.
• On success, RoundTrip passes r to the next round tripper unchanged and leaves the body open.
• The "throttle tokens exhausted" and "throttle wait complete" lines log at Debug level. A logger at the default Info level no longer writes them.
• The throttle reads the limiter's token count only when the logger has Debug enabled.
• Both lines log with the request context, the same context that the Debug check reads.
• "throttle wait complete" logs only after a successful wait.

Tests

• TestThrottleRoundTripper_ClosesBodyOnError: each of the three error returns closes the body once and sends nothing to the next round tripper.
• TestThrottleRoundTripper_PassesRequestOn: on success, the next round tripper receives the same request with the body open.
• TestClient_Retry_CanceledDuringThrottleWait: with WithThrottleEvery and WithRetry, a context cancelled during the wait returns an error that wraps context.Canceled, and the body closes once.
• TestThrottleRoundTripper_LogsAtDebug: a failed wait logs no "wait complete" line, a successful wait logs one, and every throttle line is at Debug level. Its handler drops records that lack the request context.
• TestThrottleRoundTripper_LogsRate and TestThrottleRoundTripper_LoggerTakesOneToken now read a Debug-level handler.

Follow-ups to #20

• throttle.Config returns as a deprecated type with its released fields, RPS and Burst. feat(client): add WithThrottleEvery for fractional throttle rates #20 removed it, but every client release from v0.0.2 to v0.0.7 exports it.
• TestClient_WithThrottleEvery_Burst checks each request result. The first two requests succeed, and the third fails its token wait with ErrWaitingFailed.
• TestClient_WithThrottleEvery_CanceledDuringWait no longer requires ErrWaitingFailed, because a cancel that fires before the limiter wait returns ErrContextEnded.
• A doWithin test helper fails a test when Do runs longer than a second. The burst test and both throttle cancel tests send through it, so a wait that ignores its context fails fast instead of hanging until the test timeout.

Closes #21