fix: flush buffered stdout once at end of exec to show canister call output

[lwshang]

Summary

• icp canister call --json was producing blank output because Term::buffered_stdout() was never flushed — serde_json::to_writer takes a reference and the buffer held the bytes unsent.
• Consolidated the scattered term.flush() calls (one per output branch) into a single flush at the end of exec, after all writes and after propagating any closure error.

Test plan

• New integration test canister_call_json_output deploys a canister, calls greet with --json, and asserts stdout is non-empty valid JSON with the expected response_bytes and response_candid fields — directly covering the regression.
• Existing canister_call_output_modes and other call tests continue to pass.

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