PYTHON-6111 C extension accepts BSON documents where element data overlaps the document terminator

[blink1073]

PYTHON-6111

Changes in this PR

• The C BSON decoder accepted documents where an element's value consumed the document terminator byte; the pure-Python decoder rejects them with InvalidBSON.
• Adds bounds checks to the boolean and regex element handlers so the C decoder matches the pure-Python decoder.
• No public APIs changed. All reads stay within the caller's buffer.

Test Plan

• Added a regression test with terminator-overlapping boolean and regex payloads, exercised through all decode entry points and buffer types.
• Confirmed the test fails without the fix and passes with it; the full test/test_bson.py suite and just lint-manual pass.
• just typing (mypy) crashes on an environment issue unrelated to this diff; re-run on a clean checkout before merge.

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). (No followup work identified.)

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?