[ISSUE #5411] Serve the legacy SDK gRPC protocol on the v2 runtime (port 10205 bridge) + connector plugin tests 23/23 (#5412)

* [ISSUE #5411] Serve the legacy SDK gRPC protocol on the v2 runtime (port 10205 bridge)

EventMeshGrpcServer binds PublisherService/ConsumerService/HeartbeatService on
eventmesh.grpc.port (opt-in, 0/-1 = off) and maps every call onto the v2
UniIngressService pipeline (WAL at-least-once, shared retry/DLQ):
- publish/batchPublish/publishOneWay/batchPublishOneWay -> ingress.publish (topic = subject)
- requestReply -> ingress.request with the legacy TTL attribute
- webhook subscribe -> WebHookChannel target; CLUSTERING->LOAD_BALANCE, BROADCASTING->BROADCAST
- subscribeStream (bidi) -> GrpcStreamChannel push target, ACKs ride back on the stream
- heartbeat -> GrpcClientRegistry TTL refresh + reaper unsubscribes stale clients
GrpcCloudEventMapper holds the proto<->v2 CloudEvent mapping + legacy Response envelope.
ruleGrpcProtocolHidden grants the sanctioned runtime.grpc.. adapter (TCP-rule precedent).

* [ISSUE #5411] gRPC bridge tests: hermetic mapping unit tests + real-SDK integration

GrpcCloudEventMapperTest covers every proto attribute flavor + the 3-key response
envelope. GrpcLegacyBridgeIntegrationTest boots the bridge over the in-memory
storage and drives it with the real EventMeshGrpcProducer/Consumer (the SDK as its
own conformance suite): publish, batch publish, stream subscription round-trip.

* Add unit tests for the remaining 19 connector plugins (23/23 coverage)

Hermetic where possible: webhook sinks (dingtalk/http/knative/lark/slack/wechat/
wecom/chatgpt) verified against a local capturing HTTP server; mcp (JSON-RPC
envelope), openfunction (Ce-* context headers), prometheus (single merged push),
spring (EventForwarder contract) exercised with their exact wire contracts;
webhook sources (10 plugins) POST their native callback payloads through the
lazy-bound hook port; prometheus/spring sources cover scrape + buffer drains;
external-client plugins (canal/jdbc/mongodb/rabbitmq/redis/s3/pravega) get
contract-level tests (init config-parse where lazy, commit no-op) since their
backends cannot boot in CI. Closes the "4 of 23 plugins carry the only unit
tests" gap tracked under the #5296 review.

* [ISSUE #5411] docs: legacy gRPC bridge served (opt-in); Connector Runtime test status

configuration.md flips eventmesh.grpc.port from RESERVED to served (opt-in, 1.x
default 10205); protocols.md gains the legacy-gRPC mapping table section; both
READMEs note the served bridge and update the Connector Runtime row to 23/23
plugin test coverage (drops the stale "4 of 23" wording).

* docs: make the Connector Runtime GA-criteria cell concrete

Replace the vague "GA criteria tracked under the #5296 architecture review"
wording with the actual remaining GA blockers: per-plugin real-backend
integration tests and the connector-runtime HA story. Also reference #5412
for the 23/23 unit-test coverage.

* docs: refresh remaining cross-references for the #5411 bridge and 23/23 tests

- deployment.md: connector-runtime status drops the stale "only 4 of 23
  plugins carry unit tests" (now 23/23 since #5412; real-backend ITs stay
  the GA gate)
- architecture/overview.md: protocol-status bullet + legacy-SDK paragraph
  mention the served opt-in gRPC bridge; Key-classes table gains the
  EventMeshGrpcServer entry
- quickstart/getting-started.md: port list mentions the opt-in 10205
  legacy gRPC bridge