fix(tracing): finish child spans when blocks unwind - #3207

Description

Sentry.with_child_span loses spans when the block exits through break, a nonlocal return, throw, or an exception outside StandardError. Span#with_child_span only finishes after a normal yield or in its rescue branch. The hub restores its parent scope, but the child retains a nil timestamp and TransactionEvent excludes it.

Move finalization into ensure, scoped around the block execution. Keep the existing StandardError status handling, exception propagation, and direct Span#with_child_span return value.

Four public-API regression specs fail on unchanged master: the captured transaction has zero child spans. They pass with the fix and verify that the span reaches the transaction event, the parent scope is restored, and control-flow values/original exceptions are preserved. Existing specs continue covering normal completion and StandardError status handling.

Validation (Ruby 3.4.12, committed ruby-3.4_rack-3_redis-4 matrix cell):

• Targeted API/span specs: 176 examples, zero failures
• Full core rake suite: 1,670 examples, zero failures, three version-dependent skips
• Isolated specs: 44 examples, zero failures
• RuboCop on both changed files: no offenses
• git diff --check

Fixes #3206
Related: the original exception-handling fix in #1982.

Linear: RUBY-243