[ISSUE #5231#5232#5233] Harden ConnectorRuntime against data loss; split connector-api SPI module; add plugin unit tests (#4642) (#5328)

* [ISSUE #5231#5232#5233] Harden ConnectorRuntime against data loss, split connector-api SPI module, add plugin unit tests

P0 — data-loss hardening (follow-up to #5231/#5232/#5233):
- runSourceOnce(): per-event isolation — one bad event (publish throws) no
  longer kills the source loop or strands the rest of the batch; null events
  in a poll() batch are skipped and counted
- runSinkOnce(): per-delivery ACK isolation — one ACK failure no longer loses
  the offsets/ACKs of the remaining deliveries; EventMesh re-delivers
- offsetStore.put failures no longer abort the publish/commit path
- new getSourcePublishFailures() counter surfaces skipped events to the admin API
- 4 new unit tests covering all three hardening scenarios

P1 — connector-api module split (plan: docs/contrib/connector-api-split-plan.md):
- new :eventmesh-connector-api module with the 7 SPI interfaces
  (SourceConnector, SinkConnector, EventMeshEndpoint, HttpCaller,
  ConnectorOffsetStore, CloudEventSerializer, PollEntry) — package name kept
  as org.apache.eventmesh.connector so the 23 plugins need zero code changes
- all 23 plugin build.gradles now depend on :eventmesh-connector-api instead
  of :eventmesh-connector-runtime; compile-time enforcement in addition to
  the ArchUnit guard
- new ArchUnit rule ruleConnectorPluginsDependOnlyOnSpi (FAIL mode) + canary
  negative test proving the rule actually fires
- dist-connector packaging ships the api jar in apps/ next to the runtime jar

P1 — configuration:
- eventmesh.properties gains a commented Connector Runtime section

P0 — plugin unit tests (issue #4642 follow-up):
- file: FileSourceConnectorTest (5) + FileSinkConnectorTest (5)
- kafka: KafkaSinkConnectorTest (1, MockProducer)
- pulsar: PulsarSinkConnectorTest (2, smoke)
- rocketmq: RocketmqSinkConnectorTest (3, smoke)
- file connectors gain close()Quietly() to release file handles (Windows)

Verified: :eventmesh-connector-api:build, :eventmesh-connector-runtime:build,
:connector-{file,kafka,pulsar,rocketmq}:build, :eventmesh-runtime:compileJava,
:eventmesh-architecture-guard:test (11/11) all green locally.

* Add missing Apache license header to FakePluginCanary (fix CI checkstyle + skywalking-eyes)