fix: retry sitemap fetching on error and raise when retries are exhausted

[vdusek]

Description

The try/except in _fetch_and_process_sitemap wrapped the entire while retries_left > 0 loop instead of the loop body. Any exception raised while streaming a sitemap therefore exited the loop after a single attempt, logged a warning, and silently ended the async generator — no retry ever happened, sitemap_retries was effectively dead code, and callers (Sitemap.load, parse_sitemap, SitemapRequestLoader) saw empty or partial results with no error.

This PR moves the try/except inside the loop body so each attempt is independently retried, and raises the error once all retries are exhausted instead of swallowing it.

Behavior notes

• A sitemap that keeps failing after all retries now raises to the caller instead of silently producing empty results. SitemapRequestLoader already logs and propagates this via its own error handling.
• If a stream fails after yielding some items, the retry re-fetches the sitemap from scratch, so items yielded before the failure may be yielded again. This is pre-existing streaming-generator semantics, now just reachable via the (working) retry path.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.97%. Comparing base (dc1ed61) to head (385a6b4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1943      +/-   ##
==========================================
+ Coverage   92.85%   92.97%   +0.12%     
==========================================
  Files         167      167              
  Lines       11714    11716       +2     
==========================================
+ Hits        10877    10893      +16     
+ Misses        837      823      -14     

Flag	Coverage Δ
unit	92.97% <100.00%> (+0.12%)	⬆️

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[Mantisus]

LGTM