Retry transient download failures in the validator

[krotname]

validator.py downloads the archive of every plugin in the list on every run, with a single
requests.get that has no retries and no timeout. One dropped connection on any single entry —
including entries the pull request never touched — fails the whole job and hides the real result.

This is not hypothetical: in run 30290736279
the build (Release, x64) job lost four unrelated plugins at once:

CS-Script - C# Intellisense: ('Connection aborted.', ConnectionResetError(10054, ...))
Markdown Table Editor:       ('Connection aborted.', ConnectionResetError(10054, ...))
NppEventExec:                ('Connection aborted.', ConnectionResetError(10054, ...))
pycalc:                      ('Connection aborted.', ConnectionResetError(10054, ...))

All four are hosted on the GitHub release CDN, the job died in Validate json before MSBuild ever
ran, and build (Release, Win32) and build (Release, arm64) validated the very same JSON files
successfully in the same run. Only a maintainer can re-run the job, so a contributor is stuck
looking at a red check they cannot clear or explain.

Change

Downloads go through a requests.Session whose adapter retries connection errors and the usual
transient status codes (408, 425, 429, 500, 502, 503, 504) three times with exponential backoff,
plus a 60 second timeout so a hung host cannot stall the job indefinitely.

Error reporting is deliberately unchanged for real failures. raise_on_status=False keeps a
persistently non-200 response flowing into the existing status check with its original message, and
a host that stays unreachable still raises RequestException into the existing handler.

Worst case cost is bounded: a genuinely dead URL now takes about 12 extra seconds before it is
reported, and a healthy list costs nothing.

Verification

Against a local server that injects faults, using the session built by this change:

Scenario	Attempts	Outcome
clean download	1	200
2 connection resets, then 200	3	200
2 gateway timeouts, then 200	3	200
reset + 504, then 200	3	200
resets past the budget	4	ConnectionError raised, reported as before
504 past the budget	4	504 preserved, reported as before

The connection resets are real RSTs (SO_LINGER with a zero timeout), matching the WinError 10054
the runner saw.

python validator.py x86, x64 and arm64 all exit 0 against current master with this change,
with zero errors reported.

No new dependencies: requests.adapters.HTTPAdapter ships with requests, and urllib3 is already
pinned in requirements.txt.

[chcg]

@krotname Thanks for the contribution to stabilise the download checks. Already had this on my list but didn't find the time for it.