Stop the link sanitizer from removing commercial support URLs on transient failures

[Valyrian-Code]

Continues the script reliability and test-coverage work (#1015, #1020, #1042).

Problem

scripts/sanitize_commercial_supports.py runs every Sunday and empties a listing's URL in data/commercial_support when the link looks broken; the workflow then commits the result to main. But check_url_redirects treated two ambiguous situations as a dead link:

if response.status_code != 200:
    return True            # 403, 429, 503... all counted as dead
...
except RequestException as e:
    return True             # timeout, DNS hiccup, connection reset: also dead

Since the script only ever empties URLs and never restores them, every false positive permanently removes a company's link from the site.

Evidence this is happening

All five URLs emptied by the last three scheduled runs are alive today, and all five pass this script's own check when run locally:

Emptied in	URL	Status today
1cc0f36 (Jun 7)	csgis.de	200
1cc0f36 (Jun 7)	envirosolutions.pl	200
c8910b4 (May 31)	gismexico.com	200
6b40174 (May 24)	clearmapping.co.uk	200
6b40174 (May 24)	thinkwhere.com	alive (redirect)

That points at transient conditions on the CI runner: timeouts, rate limiting, or bot protection answering the datacenter IP with 403/429/5xx.

Fix

Keep the URL whenever the check cannot give a clear verdict, which is the choice the code already makes for SSLError:

• network-level exceptions (timeout, DNS, connection reset) keep the URL and log why;
• 5xx server errors and access-restricted or transient statuses (401, 403, 405, 408, 429) keep the URL;
• genuinely broken links (404 and other 4xx) and redirects to a different domain still empty it, unchanged.

Tests

test/test_sanitize_commercial_supports.py (on the #1015 harness, all network mocked): transient exceptions and statuses keep the URL, 404 and cross-domain redirects still empty it, same-domain http to https redirects are fine, and process_file empties only flagged lines while preserving formatting. The transient cases fail on the previous code, so the suite has teeth. Full test suite green (60 tests).

Happy to follow up by restoring the five wrongly emptied URLs above in a small data PR if you'd like.

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[Xpirix]

It would be nice to have your thoughts on this @timlinux .