Use payload input options for activity complete results

[arjunrawal1]

What changed

• Replaces the activity complete --result string flag with the shared PayloadInputOptions flags.
• Updates help text/generated command code to show --input.
• Updates activity completion tests to use the shared payload input path.

Why

This makes activity complete consistent with other CLI commands that accept payload input, including support for --input-file, --input-base64, and --input-meta.

Closes #446.

Testing

• go test ./cmd/gen-commands
• go test ./internal/temporalcli -run 'TestSharedServerSuite/TestActivity_Complete|TestSharedServerSuite/TestActivity_Complete_InvalidResult'\n- go test ./internal/temporalcli -run 'TestSharedServerSuite/TestActivity_Complete_ByRunId'\n- go test ./cmd/temporal

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