PYTHON-5873 Reject whitespace in ObjectId hex strings

[GoldenEmperor1177]

PYTHON-5873

Changes in this PR

ObjectId(str) only checked that the string has 24 characters, but
bytes.fromhex() skips ASCII whitespace. So ObjectId("0123 0011")
created an ObjectId shorter than 12 bytes, and ObjectId(" " * 24) created an
empty one. ObjectId.is_valid() returned True for them, and
generation_time raised struct.error.

The fix checks the length of the decoded bytes as well, so these strings raise
InvalidId. A string that decodes to 12 bytes has 24 hex digits, so it cannot
contain whitespace, and only inputs that previously produced broken ObjectIds
change behaviour. No API change. Added a changelog entry.

AI disclosure: I used an AI assistant (Claude Code) to help find the cause,
write the fix and the tests. I read the change and I understand it.

Test Plan

Added test_from_hex_whitespace and two cases in test_is_valid in
test/test_objectid.py. They fail without the fix and pass with it. I also ran
the other tests that don't need a server (test_bson, test_json_util,
test_bson_corpus, test_dbref, test_son, test_binary, test_code,
test_decimal128, test_timestamp), and ruff, ruff format and mypy on
the changed files. I did not run the tests that need a running mongod.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?