DRIVERS-3598 Specify and test getMore span nesting in the OpenTelemetry spec

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Please complete the following before merging:

• Is the relevant DRIVERS ticket in the PR title?

• Update changelog.
• Test changes in at least one language driver.
• Test these changes against all server versions and topologies (including standalone, replica set, and sharded
  clusters).

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Context

Command spans must nest under "the corresponding driver operation span", which is unambiguous only while an operation sends a single command. For a cursor the spec never said which operation span a getMore belongs to, and no fixture exercised getMore; so both readings passed the suite, and db.mongodb.cursor_id was asserted absent in all 25 places it appeared.

Changes

• A caller-driven getMore gets its own operation span, sibling to the cursor-creating operation's span. Driver-internal iteration, where one public API call drains the cursor itself, creates no additional operation spans.
• No span is scoped to a cursor's lifetime, so a cursor that is never exhausted leaves nothing unfinished.
• A change-stream resume ends the failed getMore operation span rather than extending it.
• db.mongodb.cursor_id raised from SHOULD to MUST and added to operation spans. It holds the id the driver sent for a getMore, even when that reply returns 0; it is omitted rather than emitted as 0 for a cursor-creating command that leaves no cursor open, and omitted for commands that may operate on several cursors at once.
• Tests covering the nesting and both cursor_id outcomes with ignoreExtraSpans: false, plus prose tests 3 and 4 for what the unified format cannot express.
• Unified test format: ignoreExtraSpans applies at every level of the span tree, without which the tests' negative assertion is not enforceable.

Two decisions worth a second opinion

Neither is stated in the design document, so please confirm rather than assume:

1. Span name getMore <db>.<collection>, following the existing convention of naming operation spans after commands rather than public-API methods. Naming it after the driver's iteration method would vary per driver and make the test unassertable cross-driver.
2. Cursor id 0 omitted rather than emitted as a literal 0. This matches the existing find.yml and aggregate.yml fixtures, which omit cursor_id on commands that exhaust in the first batch, and OpenTelemetry's convention of omitting unavailable attributes rather than encoding a sentinel.

Testing

Python implementation: mongodb/mongo-python-driver#2964

get_more.json caught a real divergence there on its first run — PyMongo used a single cursor-lifetime operation span, the reading this PR rules out, and passes unmodified now that the driver conforms. Prose tests 3 and 4 are implemented and passing.