cloud: add random access support to ResumingReader

[sravotto]

This commit adds ReadAt and Seek implementations to ResumingReader and its wrapper types (limitedReader, metricsReader), enabling random access to cloud-stored files.

Implementation details:

• ReadAt creates a new reader at the specified offset using the Opener
• Seek tracks position logically without seeking the underlying reader
• Both methods implement ioctx.ReaderAtCtx and ioctx.SeekerCtx interfaces
• Thread-safe: ReadAt can be called concurrently from multiple goroutines
• Works with all cloud storage backends that support range requests

This enables efficient reading of formats like Parquet that require random access, without buffering entire files to disk.

Release note: none

Epic: CRDB-23802

[cockroach-teamcity]

This change is 

[github-actions]

Potential Bug(s) Detected

The three-stage Claude Code analysis has identified potential bug(s) in this PR that may warrant investigation.

Next Steps:
Please review the detailed findings in the workflow run.

Note: When viewing the workflow output, scroll to the bottom to find the Final Analysis Summary.

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• If the detected issue is real or was helpful in any way, please tag the issue with O-AI-Review-Real-Issue-Found
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[sravotto]

Claude bug detected:

BUG_DETECTED: The limitedReader.ReadAt() implementation at pkg/cloud/impl_registry.go:468-473 bypasses configured rate limiting by directly delegating to the underlying reader without tracking bytes read or applying rate limit waits.

As suggested, changed the ReadAt function.

func (l *limitedReader) ReadAt(ctx context.Context, p []byte, off int64) (n int, err error) {
    if readerAt, ok := l.r.(ioctx.ReaderAtCtx); ok {
        n, err = readerAt.ReadAt(ctx, p, off)
        if n > 0 {
            if err := l.lim.WaitN(ctx, int64(n)); err != nil {
                log.Dev.Warningf(ctx, "failed to throttle read: %+v", err)
            }
        }
        return n, err
    }
    return 0, errors.New("ReadAt not supported by underlying reader")
}