fix: implement Flush on trackingResponseWriter

[pinginfo]

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Description

Add http.Flusher interface implementation to trackingReponseWriter so that streaming handlers like ListenBucketNotification continue to work when the double-response guard is active.

Commit 52eee5a introduced trackingReponseWrite to prevent double HTTP responses on a single request. However trackingReponseWriter did not implement the http.Flusher interface. This caused xhttp.Flush(w) which uses a type assertion w.(http.Flushed) to become a no-op when the wrapper was active. As a result, streaming handlers like ListenBucketNotification (used by mcli watch) would buffer event data indefinitely without ever flushing it to the client, making the notification listener appear broken.

Motivation and Context

ListenBucketNotification uses a long-lived HTTP connection with Server-Sent Events. After writing each event, it calls xhttp.flush(w) to immediately push data to the client. Without http.Flusher on trackingResponseWriter, the flush was silently skipped, and events never reached the client.

How to test this PR?

1. Start a MinIO server
2. Run mcli watch myminio/bucket
3. Push file to the bucket
4. You should receive events

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (52eee5a)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here