fix(client): release SSE reader locks

[Genmin]

Summary

• release the parsed SSE stream reader in a finally block after _handleSseStream exits or throws
• keep the existing outer error path intact so disconnect handling and reconnection scheduling still run after cleanup
• add regressions for both graceful EOF and read-error cleanup

Fixes #1959.

Validation

• npm test -- test/client/streamableHttp.test.ts
• npm run typecheck
• npm run lint
• npm test

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Latest commit: f74d19a

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commit: f74d19a