winklemad has created PR #2961: PYTHON-XXXX Reject extra fields in Extended JSON timestamps in mongo-python-driver

Issue Text:
PYTHON-XXXX

1. 1. Changes in this PR

`bson.json_util.loads` now rejects `$timestamp` value documents that contain anything other than the required `t` and `i` components. Previously, unexpected fields could be silently discarded while constructing `bson.timestamp.Timestamp`.

This adds regressions for both additional fields and same-length invalid key sets, and updates the changelog and contributor list.

AI assistance disclosure: I used OpenAI Codex to help investigate the existing behavior, search for duplicates, implement the change, and run validation. I reviewed and understand every changed line and can explain and maintain the contribution.

1. 1. Test Plan

• `python3 -m unittest test.test_json_util.TestJsonUtil.test_timestamp test.test_json_util.TestJsonUtil.test_timestamp_with_invalid_fields`
• `python3 -m unittest test.test_json_util test.test_bson_corpus`
• `ruff check bson/json_util.py test/test_json_util.py`
• `ruff format --check bson/json_util.py test/test_json_util.py`
• `python3 -m compileall -q bson/json_util.py test/test_json_util.py`
• `git diff --check`

All listed checks passed.

1. 1. Checklist

1. 1. 1. Checklist for Author

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

No follow-up work is currently identified. As an external contributor without an existing JIRA ticket, I retained the template `PYTHON-XXXX` placeholder for a MongoDB employee to update.

1. 1. 1. Checklist for Reviewer

• [ ] Does the title of the PR reference a JIRA Ticket?
• [ ] Do you fully understand the implementation?
• [ ] Is all relevant documentation updated?