chore: cloudevents v2 migration

[dependabot]

Three issues introduced by the cloudevents v2 migration were caught by CI: an unsorted import block, a str | None type mismatch in str.join(), and a key_mapper callback signature incompatible with to_structured_event.

Changes

• Import ordering: sorted third-party imports alphabetically (configargparse before dateutil) to satisfy ruff I001
• get_subject() nullability: BaseCloudEvent.get_subject() returns str | None in v2; added or "" fallback in _key_mapper to satisfy mypy
• _key_mapper signature: changed from (ce: CloudEvent) -> str to (ce: BaseCloudEvent) -> str | bytes | None to match to_structured_event's expected Callable[[BaseCloudEvent], str | bytes | None]; BaseCloudEvent moved into the TYPE_CHECKING block per ruff TC002 (safe due to from __future__ import annotations)

[hairmare]

@copilot migrate to cloudevents 2.x https://github.com/cloudevents/sdk-python/blob/main/MIGRATION.md

[Copilot]

@copilot migrate to cloudevents 2.x https://github.com/cloudevents/sdk-python/blob/main/MIGRATION.md

Migrated to the cloudevents v2 API in commit e2bac3d. Changes include:

• Updated imports to cloudevents.core.v1.event.CloudEvent and cloudevents.core.bindings.kafka
• CloudEvent construction now uses attributes= keyword argument
• eventTime is parsed to a timezone-aware datetime using dateutil (now a required v2 dependency)
• _key_mapper updated to use explicit getters (get_type(), get_source(), get_subject())
• to_structured → to_structured_event
• Tests updated to reflect v2 behavior (attribute-based comparison, application/cloudevents+json content-type)