engineering: Reduce verbosity of httpsubfile in high-retry scenarios (#538)

# 🔍 Description

Clarity improvements in `crates/trident/src/io_utils/http/subfile.rs` to
reduce log noise when servers return empty (0-byte) responses:

- **Variable rename**: `received_empty_response` →
`previous_response_was_empty` — better captures that the flag reflects
the *prior* iteration's state, not the current one.
- **Trace message**: `"Successfully populated PartialResponseReader for
subfile '{}' at position {} with {} bytes available"` → `"Received a
response for subfile '{}' at position {} with {} bytes"` — shorter,
avoids implementation-detail noise.
- **Silent retry path**: `populate_reader` now retries silently after an
empty response (50ms backoff), logging only the first empty-response
occurrence to avoid flooding logs with repeated identical messages.
- **`silent` parameter**: `new_partial_response_reader` gains a `silent:
bool` flag that suppresses the "Requesting HTTP range" trace during
silent retries.
- **Code comments**: Added an inline comment above the
`new_partial_response_reader` call stating that the request should be
made silently if we've already tried before and got a zero-length
response, and expanded the `new_partial_response_reader` doc comment to
describe the `silent` parameter and its effect (when `true`, no logging
is produced).

# 🤔 Rationale

The old name and message were verbose and subtly misleading. In
high-retry scenarios (e.g. server returning repeated empty responses),
the original code would emit identical trace lines on every attempt. The
silent retry path suppresses this noise while still logging the first
occurrence so the event is observable.

# 📝 Checks

- [ ] Check
[dev-docs/manual-validation.md](/dev-docs/manual-validation.md)

# 📌 Follow-ups

TODO:

- #0000

# 🗒️ Notes

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Author: Copilot

crates/trident/src/io_utils/http/subfile.rs

@@ -1,6 +1,7 @@
 use std::{
     io::{Read, Result as IoResult},
     ops::Not,
+    thread,
     time::Duration,
 };
 
@@ -165,7 +166,36 @@ impl HttpSubFile {
         if self.reader.as_ref().is_none_or(|r| r.exhausted()) {
             // Create a new partial response reader for the next range and
             // replace any existing reader.
-            self.reader = Some(self.new_partial_response_reader()?);
+            let mut previous_response_was_empty = false;
+            let new_reader = loop {
+                // If we have already tried and got a zero-length response,
+                // make the new request silently to avoid log spam.
+                let reader = self.new_partial_response_reader(previous_response_was_empty)?;
+                if !reader.exhausted() {
+                    trace!(
+                        "Received a response for subfile '{}' at position {} with {} bytes",
+                        self.url,
+                        self.position,
+                        reader.bytes_remaining,
+                    );
+
+                    break reader;
+                }
+
+                if !previous_response_was_empty {
+                    previous_response_was_empty = true;
+                    trace!(
+                        "Received empty response when populating reader for subfile '{}' at position {}, retrying silently...",
+                        self.url,
+                        self.position,
+                    );
+                }
+
+                // If we received an empty response, we retry after a short delay.
+                thread::sleep(Duration::from_millis(50));
+            };
+
+            self.reader = Some(new_reader);
         } else {
             #[cfg(test)]
             trace!(
@@ -181,8 +211,10 @@ impl HttpSubFile {
     }
 
     /// Creates a new PartialResponseReader for the current position within
-    /// the subfile.
-    fn new_partial_response_reader(&self) -> IoResult<PartialResponseReader> {
+    /// the subfile. When `silent` is true, the "Requesting HTTP range" trace
+    /// log is suppressed — used during retries after an empty response to
+    /// avoid flooding the logs with repeated identical messages.
+    fn new_partial_response_reader(&self, silent: bool) -> IoResult<PartialResponseReader> {
         // Always attempt to read up to the end of the subfile.
         let range = HttpRangeRequest::new(
             Some(self.start + self.position),
@@ -193,12 +225,14 @@ impl HttpSubFile {
 
         // Perform the HTTP request with retries. This function guarantees that
         // the resulting response was successful.
-        trace!(
-            "Requesting HTTP range '{}' for subfile at position {}: {}",
-            range.to_header_value(),
-            self.position,
-            self.url,
-        );
+        if !silent {
+            trace!(
+                "Requesting HTTP range '{}' for subfile at position {}: {}",
+                range.to_header_value(),
+                self.position,
+                self.url,
+            );
+        }
 
         let response = super::retriable_request_sender(
             || {