fix: prevent sendRequest double-done panic

[nodece]

Fix #1497

Motivation

Under timeout/reconnect races, the same sendRequest can hit done() more than once from different paths. Since done() clears and returns the object to sendRequestPool, a later duplicate call can access cleared fields and panic with nil pointer dereference.

Modifications

• Add an idempotency guard (doneFlag) in sendRequest.done() so duplicate invocations on the same instance return immediately.
• Keep the guard raised after reset/put to block stale-pointer calls in the reset-to-reinit window.
• Introduce newSendRequest(...) and newChunkSendRequest(...) helpers and route request creation through them, centralizing initialization for pooled objects.
• Update tests to use constructor helpers where sendRequest is created in tests.
• Add unit test TestSendRequestDoneIsIdempotentAfterPutToPool to verify a second done() call on the same pointer does not panic after pool return.

[Copilot]

Pull request overview

This PR addresses a panic in sendRequest.done() caused by double-invocation under timeout/reconnect races by adding an idempotency guard and centralizing initialization of pooled sendRequest objects.

Changes:

• Add doneFlag to sendRequest and guard done() with an atomic CAS to make repeated calls a no-op.
• Introduce newSendRequest(...) / newChunkSendRequest(...) helpers and route pooled object creation through them.
• Add/adjust tests to validate done() idempotency behavior and use the new constructors.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
pulsar/producer_partition.go	Adds doneFlag, constructors for pooled sendRequest, and updates send paths to use them.
pulsar/send_request_pool_test.go	Adds a unit test asserting done() can be called twice on the same pointer without panicking after pool return.
pulsar/message_chunking_test.go	Updates chunking test helper to construct sendRequest via newSendRequest instead of a literal struct.

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[RobertIndie]

Could you help take a look the comments from Copilot? @nodece

[nodece]

@RobertIndie Done.