test(project): keep the #1851 and #1878 tests green on the KTLO legs (#1898, #1899)

[JarryShaw]

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Both tests fail on the KTLO legs on main. Those legs pass regardless, because of #1679.

• test(project): the #1851 no-requests test loads conda_vendor.py without the tomllib skip #1898: the ci(conda): nothing retires a PLATFORM_PINS entry once upstream ships the wheel again #1851 no-requests test loaded util/conda_vendor.py directly. That module imports tomllib, which needs Python 3.11. The test now loads it through the file's guarded loader, load_vendor, which skips without tomllib. TomllibGuardTests hides tomllib, runs every other test class in the file, and fails if any of them errors.
• test(project): the #1878 engine-stub tests call ast.unparse, which Python 3.8 lacks #1899: ast.unparse needs Python 3.9, so the feat(engines): type the third-party engine packages with local stubs and a per-engine _expkg type #1878 engine-stub helpers now match AST nodes directly through _named(). On 3.14 the protocol sets, _expkg annotations and members are identical to before. test_ast_unparse_is_gated_1899_unit.py scans tests/ for an ast.unparse call outside a sys.version_info gate.
• Proof on real interpreters:
  • 3.8: the ci(conda): nothing retires a PLATFORM_PINS entry once upstream ships the wheel again #1851 file went from 1 failed to 0, and the feat(engines): type the third-party engine packages with local stubs and a per-engine _expkg type #1878 file from 2 failed to 0.
  • 3.10: the ci(conda): nothing retires a PLATFORM_PINS entry once upstream ships the wheel again #1851 file went from 1 failed to 0.
  • Each new check fails with its fix reverted. The touched files, tests/project and the tier guard pass on 3.14.

Closes #1898
Closes #1899

[github-actions]

Coverage: 90.07% (unit tier, Python 3.15, 82c28016b, Unit Tests run success)

Package	Statements	Missed	Branches	Partial	Cover
pcapkit (top level)	104	2	20	2	96.77%
pcapkit/const	19595	881	2618	867	92.11%
pcapkit/corekit	2504	70	792	36	96.54%
pcapkit/dumpkit	276	3	100	3	98.40%
pcapkit/foundation	2916	164	1016	34	93.59%
pcapkit/interface	138	7	52	5	93.68%
pcapkit/protocols	17411	224	4580	189	98.10%
pcapkit/toolkit	872	177	298	8	77.18%
pcapkit/utilities	522	2	158	4	99.12%
pcapkit/vendor	4431	2341	1018	156	43.64%

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

[JarryShaw]

Verdict: GOOD TO GO at 82c28016b. In CI, the 3.8, 3.9 and 3.10 KTLO unit legs now fail no test. On main they fail three, one and one. A Sonnet cross-review confirmed the following:

• test(project): the #1851 no-requests test loads conda_vendor.py without the tomllib skip #1898: conda_vendor.py is loaded in one place only, load_vendor. TomllibGuardTests fails when a test loads it directly, when VerdictTests does, and when the skip is removed. It restores sys.modules on exit.
• test(project): the #1878 engine-stub tests call ast.unparse, which Python 3.8 lacks #1899: _named() reproduces the old ast.unparse results for every engine module's _expkg annotation, protocol set and member. Across all class bases in the repo, the new base check never disagrees with the old one.
• Older interpreters: the PR's counts reproduce on real 3.8.20 and 3.10.21, both before and after the fix.
• 3.14: tests/project passes, and so does the tier guard.

Not blocking: the ast.unparse scan misses a call in the else: branch of a version gate, a gate written the wrong way round, and getattr(ast, 'unparse').