fix(examples): retry the Wireshark sample downloads on 429, 5xx and timeouts (#1658)

[JarryShaw]

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_download (examples/generators/pcapng.py:876) now retries HTTP 429, 5xx and timeouts: 3 attempts, 2 s backoff doubling, or the Retry-After header (seconds or HTTP-date), with at most 30 s of waiting per download. A Retry-After past that budget ends the retries rather than being cut short. Other 4xx and unreachable hosts still fail at once, so an offline run is as quick as before. Each retry is logged on stderr; the SHA-256 check and the stand-in fallback are unchanged. Only the waiting is bounded: urlopen's 30 s timeout applies per socket operation, not per attempt.

New tests/project/test_pcapng_download_retry_1658_unit.py stubs urlopen and the clock: 429 then success, Retry-After in both forms, the budget and its inclusive edge, 404 not retried, 500 and 599 retried, exhausted retries falling back to the stand-in. The -0000 date test pins TZ=America/New_York, since on a UTC host the bug it guards cannot show. All 8 fail against the old generator. The 404 test fails there only on its 429, 5xx and timeout control rows, since the old code retried nothing. Also green: tests/test_support_helpers.py, tests/project/test_generators_import_their_own_tree.py, test_isort_clean.py.

Closes #1658

[JarryShaw]

Verdict: GOOD TO GO at 26c618114. Sonnet cross-review, two rounds. In round 1 the -0000 Retry-After row only failed off UTC, which CI never runs, and two boundary mutants survived. Round 2 pins TZ=America/New_York and restores it, and adds rows for a wait of exactly the 30 s budget and for 500 and 599. All three mutants now die. I re-ran the new test under TZ=UTC and test_support_helpers.py: 8 and 37 OK.

[github-actions]

Coverage: 89.66% (unit tier, Python 3.14, 26c618114, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	3	20	3	95.16%
pcapkit/const	18858	954	2366	868	91.06%
pcapkit/corekit	2366	67	760	35	96.48%
pcapkit/dumpkit	275	3	100	3	98.40%
pcapkit/foundation	2884	145	1010	35	94.14%
pcapkit/interface	138	7	52	5	93.68%
pcapkit/protocols	17166	226	4580	191	98.06%
pcapkit/toolkit	1036	168	386	5	82.35%
pcapkit/utilities	494	4	146	4	98.75%
pcapkit/vendor	4409	2342	1006	157	43.25%

Per-file detail: the coverage-html artifact of this run.