fix(types): expose cooperative-sticky rebalance methods on KafkaConsumer

[Dvir Arad (dvirarad)]

Summary

Fixes #469.

When partition.assignment.strategy=cooperative-sticky is set, the rebalance callback needs to call incrementalAssign / incrementalUnassign instead of assign / unassign. The runtime exposes them (see lib/kafka-consumer.js):

KafkaConsumer.prototype.incrementalAssign = function(assignments) { … };
KafkaConsumer.prototype.incrementalUnassign = function(assignments) { … };
KafkaConsumer.prototype.assignmentLost      = function()            { … };
KafkaConsumer.prototype.rebalanceProtocol   = function()            { … };

The stock rebalance handler in lib/kafka-consumer.js (lines 68-79) even branches on rebalanceProtocol() === 'COOPERATIVE' to call incrementalAssign / incrementalUnassign. But none of those four methods are declared in types/rdkafka.d.ts, so TypeScript consumers writing their own rebalance callback must fall back to (consumer as any).incrementalAssign(...) (the screenshot in #469 shows exactly this).

Fix

Types-only change in types/rdkafka.d.ts. Added four method declarations on KafkaConsumer, each with a JSDoc block mirroring the runtime documentation in lib/kafka-consumer.js:

incrementalAssign(assignments: Assignment[]): this;
incrementalUnassign(assignments: Assignment[]): this;
assignmentLost(): boolean;
rebalanceProtocol(): "NONE" | "COOPERATIVE" | "EAGER";

• Assignment is the existing alias TopicPartition | TopicPartitionOffset, matching the assign(...) signature — the runtime maps both into TopicPartition via TopicPartition.map(...).
• The rebalanceProtocol() return is constrained to the three literal strings the runtime can produce ("NONE" | "COOPERATIVE" | "EAGER"), so user code can do if (c.rebalanceProtocol() === "COOPERATIVE") without a string cast.
• assignmentLost() returns boolean per the JS doc (@return {boolean} true if assignment was lost.).

The original issue only mentions incrementalAssign / incrementalUnassign, but assignmentLost and rebalanceProtocol are part of the same cooperative-sticky workflow and have the same missing-types problem — happy to scope the PR back to just the two if maintainers prefer.

Test plan

• tsc -p . (the existing npm run test:types script) still passes — added declarations only.
• Confirmed each method exists on the runtime prototype in lib/kafka-consumer.js (see lines 305–308, 319–322, 342–344, 351–353 on master).
• Confirmed assignmentLost and rebalanceProtocol aren’t already declared anywhere else in types/*.d.ts.
• CI green.

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