refactor(vendor): keep the vendor variables in pcapkit.vendor (#1726)

[JarryShaw]

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Refs #1726. This follows the owner's later ruling that the vendor variables stay in pcapkit.vendor, which reverses the part of #1743 that moved them.

• Names restored: pcapkit.vendor.default.MAX_RETRY (PCAPKIT_VENDOR_RETRY, default 5, minimum 1) and CI_MODE (PCAPKIT_CI_MODE) are back under their 1.4.x names. feat(utilities)!: one config module for the tunable limits (#1726) #1743's versionremoved notes for them are gone.
• Bad values still warn: both now read through config.read_limit and config.read_flag, the newly public _read_limit and _read_flag. A bad value warns with ConfigWarning and falls back, where 1.4.x crashed on a non-integer and read 0 as 1.
• Proxies: get_proxies() reads PCAPKIT_HTTP_PROXY and PCAPKIT_HTTPS_PROXY itself, on every call.
• Removed from config: VENDOR_RETRY, CI_MODE, http_proxy and https_proxy. They never shipped, so they are deleted outright with no re-export.
• Docs: the four variables are documented on the pcapkit.vendor.default page, and config.rst links there. Sphinx warnings are identical to main.
• Tests: feat(utilities)!: one config module for the tunable limits (#1726) #1743's mutation-killing vendor tests move to tests/vendor/test_vendor_variables_1726_unit.py. Their notices, both retry loops and the per-call proxy reads are all kept.

[JarryShaw]

Verdict: GOOD TO GO at 8c525c06f. A Sonnet cross-review checked it (author: Opus).

• MAX_RETRY and CI_MODE are back under their 1.4.x names, with no versionremoved left behind. The new versionchanged notes match the pre-feat(utilities)!: one config module for the tunable limits (#1726) #1743 source.
• get_proxies() reads both proxy variables on every call. Mutants that cached either one at import were killed.
• read_limit and read_flag are public, documented and listed in __all__. config imports only os, typing and utilities.warnings, and importing it does not load pcapkit.vendor.
• config's vendor names are gone, with no re-export. No stale reader or test is left anywhere in the repo.
• Sphinx shows the same 168 warnings on both trees. In the built HTML, the envvar-PCAPKIT_* anchors are on vendor/default.html.
• My own re-run in the PR's tree: test_config_unit 36 passed, tests/vendor 142 passed, plus isort, the refactor(corekit)!: move utilities.decorators into corekit, and state the corekit/utilities rule (#1712, option A) #1735 boundary, documentation_claims and public_api.

[JarryShaw]

Verdict: NEEDS CHANGES at 8c525c06f. CI fails test_unit_tests_queue.py::TestDocsOnlyPathRunsEveryDocsReader on 3.11–3.14. The new tests/vendor/test_vendor_variables_1726_unit.py reads the docs, so it has to be listed in project-tests in unit-tests.yml. A one-line fix is in progress.

[JarryShaw]

Verdict: GOOD TO GO at e41eaf041. A Sonnet check verified the fix (author: Opus).

• The only change since 8c525c06f is one line, unit-tests.yml:1709: tests.vendor.test_vendor_variables_1726_unit is appended to project-tests' docs-reader step.
• test_unit_tests_queue.py fails on the old head and passes on the new one. I re-ran it myself: 38 passed, 505 subtests.
• A wider scan found no other docs reader missing from that step.