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.