PYTHON-6036 Vendor the OpenTelemetry unified spec tests - #2993

PYTHON-6036

Third of four PRs splitting #2964. Base is PYTHON-5947-otel-2-transactions.

Changes in this PR

Replaces hand-written assertions with the cross-driver spec suite, so the spans from PRs 1 and 2 are checked against the same fixtures every other driver runs.

• Added observeTracingMessages and expectTracingMessages support to the unified test format runner.
• Vendored 22 of the spec's 23 tracing fixtures, covering per-operation attributes and nesting for around 20 operations.
• Added a resync-specs.sh entry to refresh them.
• Span attribute values are adapted to what the shared match evaluator expects, since they arrive as plain Python primitives rather than BSON-decoded documents.
• ignoreExtraSpans is enforced at every level of the span tree, not just the top-level array, so a test can assert a span is not nested under a given parent by omitting it.

The getMore fixture is held back for PR 4, which adds the support it needs. The other 22 do not exercise getMore.

Around 4,200 of the 4,600 added lines are resync-specs.sh output and another ~150 are generated mirrors, leaving roughly 190 lines to review.

The 22 vendored fixtures match mongodb/specifications master. Two needed fixing upstream first; both are merged in DRIVERS-3597.

The tree-wide ignoreExtraSpans rule is the one piece here not yet on master: it is specified in DRIVERS-3598 (specifications#1973), still open. PR 4's fixture depends on it, since without it that fixture's negative assertions are unenforceable.

Test Plan

• The vendored suite: 46 tests pass, 23 each side, with one skip each for mapReduce, whose Collection methods PyMongo removed.
• Locally against a replica set alongside PRs 1 and 2: 164 tests pass. just lint clean.

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?