fix(log): retry transient JSON generation failures

[okt-limonikas]

Summary

This PR makes the React log viewer tolerate the short interval in which an
on-demand log JSON resource is not yet complete or temporarily unavailable.

The log JSON fetch now performs a bounded retry with abort-aware backoff for:

• JSON parsing failures caused by empty or truncated responses;
• transient connection and response-stream failures;
• HTTP 404, 408, 425, and 429 responses;
• HTTP 5xx responses.

Non-transient client errors are still returned immediately, and changing the
selected test aborts the current request and any pending retry delay.

Problem

Loading a test log is a two-stage operation:

1. Django returns metadata containing the log-server JSON URL.
2. The React application fetches and parses that URL separately.

The log server generates missing JSON files on demand. Before the accompanying
server fix, the public filename could become visible while the converter was
still writing it. A request could therefore return HTTP 200 with an empty,
truncated, or concurrently corrupted body.

The UI previously called response.json() once. If parsing failed, the query
remained in its error state and the selected log panel was empty or unavailable.
Selecting a different test and returning to the original test triggered a new
request after generation had completed, so the same log then appeared normally.

Why the failure is intermittent

The result depends on timing:

• already-generated logs are served normally;
• small logs may finish before another request reaches the public filename;
• large logs have a wider partial-write window;
• concurrent requests make it more likely that one client reads during that
  window.

This explains why the problem is more common for some tests and difficult to
reproduce in a development environment with cached JSON files.

Fix

The JSON fetch helper now makes at most three attempts:

1. the initial request;
2. a retry after 250 ms;
3. a retry after 1,000 ms.

Retries stop immediately when the request is aborted. Stable 4xx responses such
as HTTP 400 and non-transport exceptions are not retried. If all parse attempts
fail, the caller receives a clear Log is not ready error instead of a raw
SyntaxError.