fix(vendor): exit on one line without the vendor extra (#1850)

[JarryShaw]

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---

• pcapkit/vendor: record requests/beautifulsoup4 that fail to import in
  _MISSING and import no crawler while it is not empty; a PEP 562
  __getattr__ then raises ModuleNotFoundError for any crawler name or
  Vendor, naming the missing packages and pypcapkit[vendor]. The
  [socks]/[html5lib] extras stay warnings.
• vendor/main: main() writes that message as one stderr line and
  returns 1 before any target runs, instead of the console script and
  python -m pcapkit.vendor dying with a traceback; --help and
  --version work without the extra.
• docs and docstrings: versionchanged 1.5.0 for the package and the CLI.
• tests/vendor: subprocess regression tests with the packages blocked;
  comments that described the old unconditional import are updated.

Closes #1850

[JarryShaw]

Verdict: GOOD TO GO at b2e54d9f5. A Sonnet cross-review tried to falsify each claim and none broke.

• CLI without the extra. With requests and bs4 blocked, pcapkit-vendor prints one line naming the missing distributions and the extra, then exits 1 with no traceback. I reproduced this myself. Blocking only one of the two names only that one. --help and --version still work.
• Library without the extra. import pcapkit.vendor succeeds. Crawler names raise ModuleNotFoundError with the same message, an unknown name raises a plain AttributeError, and dir, pydoc and reload all work.
• With the extra installed. __all__ and the --help output are identical to base.
• conda _extern. In a simulated tree where the dependencies resolve only from pcapkit/_extern, nothing is reported missing. Without _extern the CLI exits with the one-line message. A real conda build was not run.
• Fail-before. With the old __init__.py/__main__.py, the new tests fail for the right reasons (a traceback in stderr, no _MISSING). The edited test files differ from base only in docstrings and comments.
• Notes, not blockers:
  • hasattr(pcapkit.vendor, 'EtherType') raises rather than returning False while the extra is missing.
  • The module-level import typing exposes pcapkit.vendor.typing; a TYPE_CHECKING import would avoid that.

[github-actions]

Coverage: 90.06% (unit tier, Python 3.15, b2e54d9f5, 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	2462	69	780	35	96.55%
pcapkit/dumpkit	276	3	100	3	98.40%
pcapkit/foundation	2915	164	1016	34	93.59%
pcapkit/interface	138	7	52	5	93.68%
pcapkit/protocols	17406	224	4580	189	98.10%
pcapkit/toolkit	872	177	298	8	77.18%
pcapkit/utilities	512	2	158	4	99.10%
pcapkit/vendor	4429	2341	1018	156	43.62%

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