fix: Clear Scope._transaction when span is unset

[thegunslingers]

When scope.span is set to None (e.g. after a transaction ends), Scope._transaction was not being cleared, leaving stale transaction names on the scope. Add an elif branch in the span setter to set _transaction to None when span is None.

Child Span assignments (which happen during Span.enter) do not clear _transaction, preserving the parent Transaction's name.

Description

When a Transaction is assigned to Scope.span, the setter copies transaction.name into Scope._transaction. On transaction exit, span is restored to None, but _transaction was left set. Later events (e.g. via LoggingIntegration) then inherited that stale name through _apply_transaction_name_to_event — commonly showing up as responses {model} from OpenAIIntegration on unrelated errors.

The setter clears _transaction only when span is None. Clearing on every non-Transaction would be wrong: child spans set scope.span = self while a transaction is still live.

Added test_transaction_name_cleared_when_span_unset covering Transaction → child Span → None. Lint: uv run ruff check --fix / uv run ruff format.

Issues

• resolves: GH-7774 & PY-2908

Reminders

• Please add tests to validate your changes, and lint your code using uv run ruff.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community