fix(fields): NumberField.__init__'s bit_mask shift still leaks a bare ValueError, and #829's guard is skipped when bit_length is supplied #831

NumberField.__init__ has a second eager shift that #829 does not guard, and it leaks the same bare builtins.ValueError that #828 was about.

pcapkit/corekit/fields/numbers.py:88 — self._bit_mask = (1 << bit_length) - 1. Measured on #829's head (a21a10fd9), so this survives that fix:

NumberField(length=4, bit_length=-1) -> ValueError: negative shift count   is BaseError=False
NumberField(length=4, bit_length=32) -> ok, template='>I'        (control)

is BaseError=False is the defect: a caller cannot catch it as a pcapkit error. #829 guards the shift in __call__ (reached only when bit_length was not supplied); this one is in __init__ and fires when it was.

Severity is low and deliberately stated as such. bit_length is never a callable, so it cannot carry a wire value, and all three in-tree call sites pass positive literals — pcapkit/protocols/schema/link/vlan.py:47 (bit_length=3), :49 (=1), :51 (=12). So this is reachable by a programmer typo in a new schema, not by a malformed packet. That is why it is a separate issue rather than a change to #829.

A related latent gap, same area, found in the same review: a field that supplies bit_length and a negative-resolving callable length skips #829's guard entirely — no raise at __call__ — and degrades to template='>-1s' with the error deferred to FieldBase.length's template= message. Still a ProtocolError, so not a correctness break, but one wire condition then yields two different messages depending on an unrelated constructor argument. No in-tree site combines the two today.

The fix for both is the same shape: validate before either shift, raising ProtocolError from pcapkit.utilities.exceptions and keeping the Field {name} resolved to a negative length; prefix that FieldBase.length and #829 already share, so a caller matching that substring catches every case.

Found by #829's cross-review; both measurements reproduced here independently.