bump_spdx_document crashes with 'too many values to unpack' on an spdx_id containing more than one colon #901

Description

get_full_element_spdx_id (src/spdx_tools/spdx/spdx_element_utils.py:43) unpacks a fixed two-element split:

if ":" not in element.spdx_id:
    return f"{document_namespace}#{element.spdx_id}"

external_id, local_id = element.spdx_id.split(":")

If an element's spdx_id contains more than one colon, this raises ValueError: too many values to unpack (expected 2) instead of a meaningful error.

The project already anticipates this input: validate_spdx_id (validation/spdx_id_validators.py:57-61) explicitly rejects IDs with more than one colon with a clear message. But get_full_element_spdx_id is reached from bump_from_spdx2/{file,package,snippet}.py during bump_spdx_document, which can run without that validation — e.g. the pyspdxtools3 CLI's --novalidation flag, or any library caller invoking bump_spdx_document() directly. Parsers don't constrain the ID shape, so a malformed third-party SBOM reaches this line.

How to reproduce

from spdx_tools.spdx.spdx_element_utils import get_full_element_spdx_id
from spdx_tools.spdx.model import File, Checksum, ChecksumAlgorithm

f = File(name="f", spdx_id="DocumentRef-x:SPDXRef-y:z",
         checksums=[Checksum(ChecksumAlgorithm.SHA1, "a"*40)])
get_full_element_spdx_id(f, "https://ns", [])
# ValueError: too many values to unpack (expected 2)

Expected

A clear error identifying the malformed spdx_id — consistent with the message validate_spdx_id already produces — rather than an unpack error from an internal split().

I have a fix and a regression test ready and will open a PR referencing this issue.