feat: expose RetryOption for middleware read access (#475)

[ManuelReschke]

Expose RetryOption for middleware read access

Fixes #475.

Summary

This change makes the request retry configuration readable from middleware and other external code, matching the existing public Request.RetryAttempt field.

• Export RetryOption (formerly unexported retryOption) with MaxRetries, interval, conditions, and hooks.
• Add Request.GetRetryOption() that returns the live retry config, or nil when retry is not configured.
• Document the common middleware pattern for reporting exceptions only after the retry budget is exhausted, including transport failures and the limitation that RetryAttempt >= MaxRetries does not cover early stops from RetryCondition.

Motivation

RetryAttempt was already readable from middleware, but MaxRetries lived on an unexported retryOption. Callers that want to notify on failure only after all retries are exhausted had no public way to read the configured limit.

API

// nil when retry was never configured on client or request
ro := resp.Request.GetRetryOption()
if ro != nil {
    failed := resp.IsErrorState() || resp.Err != nil
    if failed && ro.MaxRetries >= 0 &&
        resp.Request.RetryAttempt >= ro.MaxRetries {
        // report once after final failure
    }
}

Notes:

• The returned *RetryOption is the live option for this request; treat it as read-only unless you intentionally mutate retry behavior.
• RetryAttempt >= MaxRetries detects budget exhaustion only. Retries may also stop earlier when a RetryCondition returns false.
• Request-level SetRetry* and client-level SetCommonRetry* continue to clone options onto each request as before.
• GetRetryOption does not create a default option; only the existing setters do.

[imroc]

Clean PR. Exports RetryOption and adds Request.GetRetryOption() for middleware read access, addressing #475.

Verified locally: go build ./..., go vet ./..., go test ./... all pass (22 packages). CI is green.

Review notes:

• Type rename retryOption → RetryOption is safe (was unexported, no external consumers).
• GetRetryOption() returns the live pointer (not a clone) — documented clearly with the read-only caveat. Reasonable design choice that also allows middleware to adjust retry behavior if needed.
• Returns nil when retry is not configured (correct — doesn't create a default like the internal getRetryOption() does).
• 7 new test cases cover nil, request-level, client-level, override, middleware use case, transport errors, and no-report-on-success. Thorough coverage.
• Does not touch internal/http3/ or internal/http2/.

No blockers. Merging.